there are a number of ways.
some people like to mask the whole canopy off then paint the frame by hand.
one way of doing it is to cover the whole thing in strips of foil, burnish it down and with a very sharp, brand new blade cut around the frames so you are left with the glass parts covered. a good rub down and you shouldnt get any seeping.
you can also try this with
tamiya masking tape or a similar low-tac tape, but if you do this, go around the edges with a liquid mask such as maskol from humbrol.
yet another way is to use just liquid mask, you can either try painting the mask on by hand onto the glass parts (if you feel you have a steady enough hand) or you can cover the whole thing and after every section run your finger over the raised parts (the frame) this should remove the liquid mask only from the frame whilst keeping the glass parts covered.
and last but not least, you can buy a pre-cut masking set. eduard make some good ones but they are not always 100% accurate and will sometimes need some liquid mask around the edges. these pre-cut masks are very very thin and quite easy to manipulate. i love using them personally.
here is one for the JU-52 (desinged for the
revell/monogram 1/48 kit.
http://www.hannants.co.uk/search/?FULL=EDEX070
let us know what you choose and how it turns out!!!