Q: Is it okay to pre-set the corporal or corporals (if more are needed for the amount of vessels) on the altar, for instance, before mass begins? Or does it need to be placed at the same time as the vessels are brought to the altar during the mass?
A: Here’s a recent post on a related topic: https://paulturner.org/items-on-the-altar/ .
The corporal(s) go onto the altar during the preparation of the gifts. They prepare for the Liturgy of the Eucharist.