Here is the digital Build Book I made for Eris Morn when I competed at Tucson Comic-Con. Build books show the work a cosplayer has put into their build, and show contest judges everything you did that you can’t really explain in the few minutes you get at pre-judging. They also prove you are the creator of the cosplay.
I used Canva to make mine look polished, because I love pretty books, but the important thing is to use lots of pictures and a few explanations. Out of my hundreds of progress pictures, these were the clearest and most useful.