This is a very simple design and so they were not difficult to make, I just needed the idea. This is the standard flat friendship bracelet style. In the order of how you want the colors to appear in the bracelet you make two knots with the beginning string on each of the following strings from left to right and then repeat with the next string on the outermost left. That is a very bad explanation so here is a better tutorial of the basic idea: http://www.dltk-kids.com/crafts/friendship/mbracelets.htm
The bracelets were made with 4 strings folded in half to give us 8 pieces of string to knot with: 3 of the base color and 1 of the accent color. The pattern I used was: 2 rows base, 1 row accent, 2 rows base, 1 row accent, 2 rows base (and repeat).
The following colors were used for the houses:
Gryffindor: Scarlet and Gold
Ravenclaw: Blue and Bronze
Slytherin: Green and Silver
Hufflepuff: Yellow and Black
If there is any confusion (which I'm sure there will be) or if there are any questions, feel free to leave a comment and I will address it!