When to move the chains

[stegenref]

I was working a 7th grade game the other night and we had a situation where we had a dead ball personal foul, after the play was over and where team A had just gotten a first down. This has always confused me, so I'm hoping someone on here can explain it to me to where I can understand it...

Since A had gotten a first down before the dead-ball personal foul, they're entitled to the first down. But what confuses me is, do you set the chains first and then apply the 15-yard personal foul (but leave the chains) to make it first down and 25 yards to go, or do you move the chains back 15 yards as a part of the 15 yard penalty and just make it 1st down and 10, from a spot 15 yards behind the end of the run.

Please reference page numbers in rules book or mechanics manual...