Making the first warp
First you need to check your pattern, so you know how many threads to cut and in which colors. For the first braid, Lace Bend Round, you need 8 threads: 4 light and 4 dark. If you are planning to start with another pattern, check the requirements on the pattern page.
The exact length of the warp is up to you. I suggest cutting the yarn to the length of 100-120 cm. That is long enough to enable you to learn a pattern, but not long enough to be difficult. As I assume you are braiding solo, do not cut the lengths of yarn longer than 150 cm, it just won't work out. All your pieces of yarn need to be the same length.
When you have cut your lengths of yarn, fold them in two and knot the whole bundle together at the open end. Before you tighten the knot, make sure all loops are the same length and undo the knot if adjustments are needed. Having loops of different length will cause unnecessary problems while braiding.
When you have knotted together the warp, tie your cord/string around the base of the knot. Your warp is now ready. Fasten it to your fixed point with the cord.
Making a departed warp
Some patterns call for departed (bicolor) loops. For example your pattern may call for five departed loops. In that case you need five light and five dark shorter lengths of yarn. For example, if you normally cut 120 cm long pieces, now you need 60 cm + 5-10 cm, i.e. each length of yarn should be abt 70 cm long. Then you knot one light and one dark yarn together and treat the resulting length of yarn as one. When you've finished, you should have five bicolor lengths that can be folded and knotted into a warp.
Now you are ready to proceed to the next lesson, Reading a pattern.