Hi, Shoop - Without an airbrush you're left with either tape masking or brushing. Tamiya tape is my most often used, blue painters tape works well, it can be cut into very narrow strips if needed. Masking fluid is another way, but not always the best results.
Some guys use paper cut to fit, then dampened in order to adhere. Just be sure to blot the surface to be painted until dry. I've done it, it can be made to work, but the edges can be prone to lift with pressure of propellant.
There are masking agents available that are reuseable, very thick and can be rolled into "worms," leaving no residue behind. A search for masking agents should lead to a manufacturer, I think AK might be one, Mig for another.
I'm an airbrusher, but if I need really fine lines I use Tamiya tape, or the blue tape from Scotch. Pretty much foolproof.
Hope you find something that works.
Patrick