Carl,
I'm afraid there isn't any easy way of masking, it's just a matter of systematically working through it. I'm always aware that overspray will find the slightest chink! I sometimes use clingfilm to cover large areas that I don't want to get any paint on, using tape to seal the edges.
Before masking make sure that the surface is clean & dry - if over another painted surface, I allow at least 24 hours before masking. Remove the masking tape when the new paint is touch dry, if you leave it a long time, then you can lift the paint. Make sure that the masking tape is burnished down at the edges - I use a paintbrush handle. You can try to seal the edges with varnish, to prevent bleed through/under, but that can cause more trouble.
This how not to do it!!! Poor surface preparation - poor choice of primer - insufficient time to let the basecoat dry before masking - result, a lot of bad language!!
View attachment 387683
Just about recoverable - removed as much loose paint as possible, feathered all the edges, reprimed etc etc - I lost heart, and it still ain't finished!!
Dave