You will need
1) The tank model.
2) A pin vise
3) A pin with a round head.
4) A wire cutter or pliers
5) Superglue optional.
Drill a hole in the middle of the turret plug, the same size or slightly smaller than the pin dameter.
Snip the pin about 3-5mm from the ball head. This should be about the same as the depth of the hole drilled in the turret.
Do a test fit. I usually find it pretty snug which means I don't need superglue later. If it is too tight, enlarge the hole.
Put the turret plug through the hole in the tank hull. Insert the pin into the drilled hole in the plug. Don't push it in
all the way in, you need to leave a little play. If you push it in till the turret is tight against the top of the hull, it
won't rotate easily. Tanks with uneven hull tops will need more clearance.
You can paint before or after mounting the turret, either way has pros and cons.
Some examples of completed tanks.