My first custom build

Hi all, I am attempting my first custom build of a rifle. The overall picture is a custom barrel from Hart Barrels, a mcmillan a-5 stock with a remington 700 action in 338 lapua. My experience with firearms until now is that I inherited a decent cache of rifles from a deceased relative and i started target shooting and hunting only about 2 yrs ago. I dont know much about rifling twists and how long the barrel should be, but i have used all the components (in different rifles that i own) and i think that is what i want to put together for my first build. I am not really sure where to go from there. I am open to any advice/tips/guidance. Thanks.