Chombi

Depending on the brand/model/age of the freewheel, most of them have holes at the back of their bodies where the pawl/springs pivot on. I usually first flush out the old grease and dirt from it by blasting it with a lot of WD40 over a basin and then soaking it a bit (Mineral psirits also work well to clean out old stiff grease). Once you think the bad stuff is out, dry it throroughly with paper towels, get a tube of your favorite water resistant grease, like Phil Wood or Park and press the opening of the tube against one of the holes on the back of the frewheel body then spin the frewheel as you push in the grease from the tube do it for all the holes in the back of the freewheel body. Do so until you see the grease starting to come out of the freewheel body seam at the back and front and continue to spin the freewheel to distribute the grease evenly inside the freewheel so it reaches all the bearings and races too. Wipe off all the excess coming out of the holes and seams and make sure that the pawls are working properly and locking the freewheel in the drive direction roatation and re-install it on you rear wheel hub.
You can also open up the freewheel to do a more extensive service/cleaning of it, but only if you feel that something might need to be replaced or adjusted inside , like bearings, pawls and springs.
I've been doing it this way for many years and it has always worked well for me You don't even have to bother to remove the cogs in most cases, so you get to also clean your cogs at the same time.

Chombi