Apostle questions

[Typhous]

Do we know anything about the process of becoming an apostle besides the need for a behelit and a sacrifice?
Are the parties involved taken to the interstice?
Do apostle hopefuls have to give up their flesh like griffith did?
Are the sacrifices branded?

 

[Aazealh]

All of your questions are answered at the end of the Black Swordsman arc, the first arc of the story. My recommendation to you is therefore to read that part of the story. Generally-speaking, reading the manga is the most important thing one should do in order to understand the story of Berserk.