A crossbow string can be made using B-50 dacron string that you get from any archery supply store, it should come pre waxed to slow down wear and tear and it will be easy to make the crossbow string.
A string for a 45-50lbs crossbow will require a 16 or 18 strand string. If you have a crossbow that is much more powerful I would recommend commercially made bowstrings just to be safe. Begin by cutting 18 strands of dacron string. Many jigs have been made to quickly figure out exactly the length of string you will need but unless ur making several or want to save as much as possible then cut the strings several feet longer than the end product needs to be. If you want to make a jig simply google bowstring jig and plans will be abundant.
You start out a a technique commonly referred to as a flemish string twist. Seperate the strings into two nine string bundles. Roll between your thumb and pointer finger foreward and then in one motion switch to the other bundle and rotate it in the opposite direction and it will create a two bundles that rotate against each other.If you want to make loops to fit over the nocks as opposed to using a knot then start braiding about 5 inches after the start of the string and fold it over and continue braiding to create a loop.The opposite side of the string you will have to tie on using a bowyers knot.
You can service the string by wrapping a layer of a single stand over the area after the loop and where the arrow will be nocked to prevent wear and strengthen the loop.