Well, here are a few pointers to get you started.
1. Rough up the seam/area with a sanding stick to allow the putty to 'grab' better.
2. Don't apply the putty too thick. It's better to put on a couple of thin coats than one thick coat. You don't want to go more than 1/16" thick.
3. Use an old Exacto knife blade to apply the putty and smooth it out before it sets.
4. Depending on what type of putty you use, make sure you let it sit and cure for an appropriate length of time so that you don't end up with it shrinking after you've already painted over it.
Hope this helps.
Ray