Most bearings in this type of application are rubber sealed, see farther down for shielded.

Use a eyeglass screw driver to pull the seal lip off the inner race then insert a grease needle and give it a pump or two. An alternate method is to pop the seal out of the outer race then apply grease to the bearing. In either case you only need to do one side on a single row bearing because most idlers / alternator bearings are single row. If double row, like some AC clutch bearings, pop a seal on each side. If the bearing seal gets slightly damaged it usually isn't an issue.

Some bearings are shielded. This is more or less a steel washer that snaps into the outer race and is close to the inner race. With these I pry up the inner race side then pump grease in.

In theory, old grease should be removed as mixing greases " could " cause a problem but I haven't had any issues over the years.

Find any sealed bearing and practice / just pry the seal off to see what you are up against.