Can anyone shed light on payback day usage?

I have 2 payback days coming and I want to use them for a 4 day trip that starts Jul 30th. The way I read it, it'll pay me for the first two days of the trip, which is what I want since they're worth 14 or so hours.

Questions: 1 - Does there have to be positive staffing to use PB days? 2 - Are they processed on the 20th like the rest of July PCS or will they go as soon as I put them in? I don't see payback days on the list of PCS steps.

Thanks!