TIME INVOLVED
This hand loom, made by Norwood Looms, is made of cherry and one of the looms that I weave on (no electricity). Please see my comments on Harrisville Designs Looms in the My Favorite Links section. The yarn on the loom is called a "warp" and the entire process of loom preparation is called "warping the loom". As you can see, there is no warp (yarn) currently on the loom. Following are the steps for warping a loom, but the order may be different for other weavers.
The yarn I'm using for this project is 25% silk/75% Cotton. The yarn was originally white so I hand-dyed it, wound it into a skein (see "Spinning Process" for how to wind a skein) and then placed it on my yarn swift. From the swift, I wound it onto spools using my electric spool winder. 
I use an electric spool winder to wind the yarn for my project onto a series of spools. The thickness of the yarn will determine the number of spools needed. The thinner the yarn, the more warp ends (strands) needed, the coarser (thicker) the yarn, the fewer warp ends needed. It can take me from a few minutes to many hours just to wind enough spools for a particular project. This yarn winder has a counter which measures in feet and I have wound as much as 2,000 feet per spool if I know I will be using the same yarn again for other projects.
This is the spool rack that I use. Once I have enough spools of yarn for a project, the spools are placed on the spool rack. This particular rack holds up to 120 spools and as you can see, I can vary the yarn colors as needed. 
This is a tension box. The spool rack is placed 4-5 feet behind the tension box and each warp end from each spool is threaded through the tension box and then tied onto the warp beam. The sectional warp beam loom has dowels spaced 2" apart (sections, hence the name). Sectional warp beams also come in 1" sections which require less spools, but take longer to wind on because there are more sections. I have another loom that does have a 1" sectional warp beam so the number of spools needed also depends on which loom I'm using at the time.
This is the back of the loom which has a 40" weaving width. Each 2" section of warp is taped in place so the sections will not become tangled. Though the yarn is now finally on the loom, the warping process is still not finished.
I unwind one taped section at a time, then move to the front of the loom and each warp end of yarn is threaded through a metal "heddle". You can see the "eyes" of the heddles in the photo on the right side of the warp.
After threading each heddle, I then "sley" (pronounced slay) the reed which is a comb that is the weaving width of the loom. It has has slots (called "dents) which also have to be threaded using a sley hook (shown on warp). I use my fingers to thread the heddles.
Here, I tied the warp ends together to keep them from slipping out of the reed and heddles. The reed assembly is called the "beater". Below the beater is the the "cloth" beam.
In this photo, you can see how the warp extends from the sectional warp beam on the back of the loom, through the heddles and reed, over the "breast" beam and is tied onto the cloth beam. 
My last before weaving step is to "tie up the treadles". The treadles (peddles) are tied to the harnesses in a particular order depending on the draft (pattern) I'm weaving. I have not discussed harnesses (this loom has 4, my other loom has 8), but I can weave on as few as two depending on the draft. The draft tells me which heddles to thread, which harnesses are needed and which harnesses are raised by treadling. 
Finally, I can start weaving. In this case, I am weaving scarves with a cotton/silk warp that I hand-dyed. The weft is a linen yarn, called flake because of its thick/thin appearance. This is also referred to as a "textured" yarn.
As the fabric is woven, it is wound onto the cloth beam which is under the front of the loom and can be seen by looking under the woven fabric. I continue weaving until I have enough of the particular item that I am weaving, in this case, my multi-use towels.
After I have finished weaving, in this case, scarves, they are removed from the cloth beam, cut, the fringe is tied and then they are ready for wet finishing which means they will be washed and line dried.
This is another scarf using a different color warp and weft.