GENERAL PROCEDURE
- Prepare the dough.
- Take out the required amount, a small piece of dough.
- Say the berakha if it applies.
- Designate that piece as Challah.
- Handle the separated piece according to the practice where you live.
Thresholds and the conditions for the berakha depend on the halakhic opinion followed and on the type of flour: ask your Rav.
Personal case: ask your Rav.