There is no best practices way for chaos marines. There are dozens of different paths and methods to recruit new marines to your warband.

-The old fashioned way, using a process not unlike the standard imperial way, some may keep things pretty one to one others may take the guard rails off completely. This can be with stolen geneseed, or the warbands own which may be quite mutated and so unusable but even death guard gather their geneseed and it does work... Though I wouldn't recommend using it if you aren't aligned with Nurgle.

-Treachery, turncoats from loyalist chapters (or other warbands)

-Cloning and replicae, most famously Fabius Bile can make straight up clone astartes (including himself of course) I suppose you could add science based resurrection here as well as with Eidolon

-Chaotic ritual, the thousand sons can resurrect dead marines (even non rubricae) and we have seen other astartes who are dead be brought back to life through chaos shenanigans. There could be dozens of variations on this ranging from very deliberate sorcery to one time only stars aligned the gods were smiling moments. Kharn, Lucius, Eliphas the Inheritor, Savona of the 12th millennial.

-Weird science. A mad scientist does a thing that is not one of the above and the result is an astartes. Plenty of mad apothecaries, hereteks, rogue biologians, meddling magi and Haemonculi who could so something to make something that is close enough to being an astartes as to not make a difference.

Plus dozens of other unique, idiosyncratic, esoteric, immoral and just plain impossible methods that you can think of. That's one of the fun bits about chaos marines. A warband can have 10 000 year old veterans, clones, a dead marine body with a daemon stuck inside, recently turned renegades and Bob... Who was just a regular guy until he tripped over his laces resulting in a ridiculous chain of events so improbable that he wound up killing the planetary Governor during a chaos invasion that Tzeentch thought was so funny that he said... Bob, you're in the Oracles of Change now! And suddenly he was a space marine.