You are going to want to start with Research.
Start by picking a specific armor set or style that you want to build, narrow it down. You mentioned the Mark V, but which Mark V, from which game? Halo CE? Halo CE:A? Reach Mark? Halo 4/5 Mark V? Each iteration of the Mark V is very different from the other, so, so you need to be specific about what exact armor set you want to build. Then start gathering reference for that specific set.
Once you have picked which Mark V, search the site and start reading through W.I.P. threads for that armor.
Also, Check out the tutorial index:
Halo Costume Tutorials
Read as much as you can and see if a specific build method sticks out to you. Pepakura. Foam. 3d printing. Then research that method. Familiarize yourself with it and see what others have built in that style. Remember that no one technique or material is objectively better than another. Each has its own pros and cons, tips, tricks, and techniques, and amazing results can come with all of them.
If you want to build your own costume, you can then go in the Armory on the site and download the files for the armor you want to make. Files are organized by Game and then armor set, and all pieces for each armor set will be included in one .Zip file. Then start building. You are going to make mistakes but until you actually start you are never going to learn and grow as a builder.
Tutorial Index