The simplest and most...well...direct way to warp a loom is the direct warping method. This method has a lot going for it. It does not require a lot of equipment, it's perfect when you do not need a lot of warp length for a project, there is little risk of your warp threads getting tangled, and it can be used on any loom, though I think it lends itself better to rigid heddle looms. It is particularly nice when you have a lot of warp color changes on a small warp space. I will describe the technique and then outline a few of the pitfalls, too.
Brief Description of Method
With the direct warping method, you use your loom's back beam and a peg to do your warping. You measure from your back beam toward the front of your loom and continue measuring until you get to the spot that is equal to the length of warp you desire. At this measured spot, you clamp your peg. For example, if you want to make a warp that is three meters in length, you would measure straight from your back beam (in the direction of the front of your loom) out three meters in an unobstructed path. At the three meter mark, you would secure your peg to, well, something. I've used step stools, the edges of tables, bookshelves; it's really whatever is either currently in that spot or something that you can move to that spot and be confident it will not easily move from that spot. I would say this technique is much easier with rigid heddle looms, as you can move these smaller, lighter looms around to more convenient locations for peg clamping. However, you also need to secure your loom! You don't want your loom to slowly inch its way across a table while you're warping it so that you last warp threads are a foot shorter than your first warp threads. Many rigid heddle looms come with clamps (for both the loom and the peg) and a peg, as well as detailed instructions for this method.
With the direct warping method, you secure your warp thread to your back beam, make a small loop around your finger and pull the thread toward your rigid heddle or heddles in the shafts of your loom. Using your warping hook, you pull this looped thread through either a slot in your rigid heddle or through a heddle in the desired shaft of your loom and continue pulling until you reach the peg. You place the loop around the peg. You should then have what looks like two threads running from the back of your loom to your peg. Walk back to your loom, and this time making sure to pull from under your back beam, repeat the process with another loop around your finger. *This would be the method if you want one thread in each slot and hole of your rigid heddle. You warp all the slots, wind your warp on your back beam, then cut your loops and move one of the two threads in each slot to the hole adjacent it. It takes a little more planning if you are using a loom with shafts- you will need to plan on which heddle in which shaft should remain empty for you to later move the warp thread. You would need to leave these heddles empty as you move across your warp, even if you are changing shafts with your loop. To be honest, I have never used the direct warping method with a four-shaft or greater loom. You would also need to make different plans if you want more than one thread in each slot or hole in a rigid heddle. (Like if you are holding your thread double for the warp, you would need to warp the slots AND holes as you walked back and forth from your loom to the peg.)
Things to Keep in Mind
This is the quick and dirty explanation of the direct warping method. Like I said, it is really great for rigid heddle looms, you can easily make warps with more than one color and it's great exercise. However, there are some pitfalls. Once you start warping, you sort of need to finish. That is unless you have a space in your house where threads three meters in length strewn across the room don't get in the way. Also, small children tend to like to tangle themselves in these warp threads. It is not the easiest way to warp a floor loom or a loom with multiple shafts, though it can be done. It is great for smaller projects, but I would caution using it for very large projects that have a lot of warp threads. All of your threads need to fit on your peg, and when you start to get in the 300's for warp thread count, it can get crowded. (Not to mention walking back and forth for 300 warp threads can be a lot.) Also, if your peg or loom moves, you'll have a little trouble on your hands. Not insurmountable, but it is something to pay attention to.
This is a method for immediate warping on the loom. The really nice thing is that once it is measured and threaded, you can get straight to weaving. The warp threads are measure right on the loom, so there is no transfer of warp threads in this method.
Indirect warping is when you measure your warp out using something like a warping board or warping mill and then move this measure warp from your board or mill to your loom. You can use this method to measure out warps for multiple projects and store the warps for a later time. Typically, you can take your time measuring the warp threads because they are nicely contained in a smaller, more compact space. I have a tutorial on using a warping board. Warping mills work in a similar way but reduce the amount of reaching because they spin, therefore reducing the demand of movement from you. The indirect method can be used easily for any type of loom. I also have a quick tutorial on warping a small rigid heddle loom with the indirect method and plan on making one for a four-shaft loom as soon as I finish my current project. This method is really nice for super long warps and also for warps that have a large number of warp threads. This is my preferred method of warping, though I certainly use the direct method, too.
Brief Description of Method
I will be very brief here, as there are various ways to warp a loom using this method because the warp threads are free to be moved. Some people enjoy warping from the back of their loom to the front, and others like going from the front to the back. I'm a front to back kind of gal, but one method is not better than another, it's just preference. The key points that make this an "indirect" method are these:
With the indirect warping method, you measure out your warp threads ahead of warping your loom. It can be intimidating at first, but once you try it out, it is really a great way to warp a loom. Let's say you need seven meters of warp. Using your warping board, you find a path that is seven meters long without doubling back or crossing the path on the board (or mill). Then, you grab your warp thread and go to town moving up and down this path. (You create a "cross" at one end; please see my tutorial for warping boards for more details on this.) A trip down your path is one warp thread, a trip back up is another warp thread, so each completed trip gives you two warp threads. You tie choke ties in the necessary places along your measured warp, then carefully removed it, braiding it as you go. This braided, measured warp can then been stored for later use or carried to your loom to begin the actual warping process.
Things To Keep in Mind
The start-up for this method is a little more costly, as warping boards and/or mills can be expensive. You can find alternative means to the commercially available options- people have made bookshelves, the bottoms of chairs and paint cans into warping boards. My husband actually made my warping board, saving us a few bucks and adding a ton of sentimental value. A quick search on the internet can give you some ideas on creative alternatives to the "official" boards and mills. This method also creates a little more waste in your warp threads, as the warp has to be tied on both the back and the front of your loom, so keep that in mind when planning your final warp length. Sometimes, particularly on rigid heddle looms that you have planned shorter warp lengths, smaller warp thread counts, and simple weave structures it is easier to just use the direct warping method. Also, attention has to paid to that cross I mentioned earlier when you actually do sit down to warp your loom. If you mess up the cross, your threads may get tangled in the most hopeless of fashion. That means if you have very large bundles, you need to be prepared to sit and warp that bundle...don't put it down! In my house, large bundles are impossible because as soon as I sit down, some emergency happens and I need to get up. To get around this, I make small bundles, even if my warp is all one color. This reduces the amount of time I need to sit in front of my loom- I can warp a lot or a little.
I hoped this cleared up some terminology. Don't feel constrained by the categories, though. I have seen people use warping boards fastened to tables to do direct warping! So clearly, with a little imagination, there can by hybrid methods that work for you.
Warp speed ahead!