Set it to the correct temperature and put it in simple as that.

Just add the heater!
if it is preset for tropical fish like mine than its great, if not, then set it to the correct temperature, that shouldnt be a problem at all.

Assuming these are tropical fish, adding the heater is not a problem.

It is not an instant heat situation where the temp will change drastically in a short span of time. Aquarium heaters take a bit of time to get the temp up to whatever you’ve got it set at, so adding it while the fish are there is not going to create an issue.

Good luck.

First we’d need to know what kind of fish you have. If they’re goldfish they absolutely should not have heat.