Back to loom pictures.

One of my tabletweaving books has a short summary of a warp-weighted loom described in Luther Hooper's Weaving with Small Appliances: Tablet Weaving. I have made several of these looms so far; the concept is quite straightforward and works very well for me.
(A copy of Hooper's book can be found here, in PDF format.)

The main feature of the loom is that the warp is not tied off or fastened in any way but hangs freely from the end of the loom. The end of each cord is tied to a small weight to maintain tension. This arrangement has two distinct advantages:
1) the tension of the warp remains comparatively steady even as the twist builds up, and
2) when necessary the build-up in each cord can be easily unwound.

A basic loom of this kind can be put together in less than an hour. It requires one long piece of wood, four long screws, tools, and some nails. I usually use a 1.8m plank, about 2cm thick (if it is too thin the weights will cause the loom to bend in the middle). Cut off two sections of 12cm to 15cm from the ends; these are screwed (and glued, just to be on the safe side) to the each end of the remaining piece to become the uprights.

If you use large tablets or if you intend to work with a very light tension, the uprights should be taller to allow space between the tablets and the base. Most of my tablets are around 7cm to the side, and uprights of 14cm total height (i.e. about 12cm above the base) work very well.

Hammer a row of nails into the top of each upright. On the one side the nails are used to tie down each cord when the loom is warped, on the other side the nails act as warp spreader to keep the threads apart. And there's your basic loom...

A slightly more advanced version which does not use nails can be made by using two pieces of plywood with vertical grooves cut into the top:

These pieces are bolted to the uprights, and can be exchanged for other pieces with larger or smaller grooves for different yarn types. The holes can also be used for other attachments, such as a wider warp spreader (in the photo of my loom the cords are very close together and wind around each other occasionally). It also helps to round the end of the upright over which the threads hang so that they slide more easily.

A More Complex Loom
A more complex variation is a variable-length loom. My original idea was to have a loom that could be collapsed for transport, and expanded again to its full length on arrival. With a bit of care and untangling of threads on arrival it kind of works...

The loom is made in the same manner as described above, except that each upright is fastened to a separate short plank. These two pieces in turn are bolted to a long base with countersunk holes; I used wing nuts to fasten the bolts to remove the need for tools, but the nuts do tend to get in the way a bit.

The holes in the two short pieces are aligned so that they can be bolted one on top of the other, making a short loom. This is handy on occasion, but on the whole is unnecessarily complicated. It is much simpler to just have two separate looms, one short and one long.

Weaving Long Bands

I use a very simple mechanism to weave long bands on a warp-weighted loom. When warping the loom I cut the threads for each cord to the desired length, and wind the surplus onto a simple metal spindle made from stiff galvanised wire. The small hook at the top is to stop the spindle from unwinding itself; the large hook at the end is used for the weight. This also removes the need for actually tying the cord to each weight. When the weaving reaches the end of the loom, the finished part can be moved up and bolted down between that handy grooved plate and the uprigh. Enough yarn to stretch along the loom and hang over the edge is unwound from each spindle, and weaving can continue. I usually tie the cards together during this process to stop them from shifting around.

A Note on Weights
I use standard fishing weights varying between 2 ounces (e.g. for fine 60/2 silk) and 5 ounces (for heavy wool) per cord. On a short loom, the weights should be slightly less since there is a shorter stretch of yarn to distribute the tension on. If I use 3 ounce weights on a 6-foot loom, I drop to 2-ounce weights on a 3-foot loom.