How to prep??

Hey everyone! I am a first time mom due in about 9 weeks. I have a stash of Alva pocket diapers that I'm needing to prep for babies arrival. I have a few questions.

First, I plan to use disposables until she can fit the one size Alva's I have. Does anyone have any experience with how soon that may be?

Second, I really want to prep before baby gets here but because I'm waiting to use them, I'm not sure when I should prep?

Lastly, How do you prep? Is it just a normal wash?

I plan to use tide original or Foca.