Forgotten Ancient's first ability will resolve before the spell that caused it to trigger. Putting a +1/+1 counter on Forgotten Ancient is optional.

Forgotten Ancient's last ability doesn't target any creatures. You choose how many +1/+1 counters will be moved (and onto which creatures) as the ability resolves. Notably, once the ability starts resolving and you make these choices, no player may take actions until the ability has finished resolving.