You don't need to prep microfiber inserts, pockets, or covers. These you can wash with your regular diapers before first use.
Two Ways To Prep
Washing - Common advice is to wash on hot for 3-5 cycles and drying in between. I did that the first time I prepped (they were prefolds), but I couldn't figure out how drying helps strip oils, so the next time I prepped (inserts and fitteds), I did 4 cycles of hot wash with no drying in between. I think I had 30 inserts and 5 fitteds then. Both ways worked fine, and I saved time and utility by only putting them in the dryer after the last wash.
Boiling - This is the faster way to prep. Just boil for 30-45 minutes and then run through one cycle in the wash. Note that you should only do this if what you're prepping does not have snaps or PUL. There's debate whether you can do this with "blends" - like inserts that have microfiber inside. Some have done it with no issues, some say not to do it because microfiber is synthetic and could melt.
- After prepping, your diapers may not be fully absorbent yet. It's okay to use them and wash them with your other diapers.
- Cotton flats only really need to be washed once to prep. I haven't tried bamboo flats, but I would guess they only need 1 or 2 hot washes to prep.
- From my experience, charcoal bamboo inserts take the longest to prep, probably because of the outer fleece-y layer. After a few more cycles in the wash (between uses), they should reach their full absorbency.