I have not found that it is normally required to clean brass before painting it, most brass comes clean. Unless, that is, you have been fondling it with your pizza-grease stained hands. I have had little problem spraying a light coat of enamel, either straight out of the rattlecan or airbrushed. If I am painting with acrylics I will most likely shoot a light coat of primer (PlastiCoat from WalMart or American Tradition from Lowes) before airbrushing.
Spray the parts right on the fret. I've tried the route of forming them before painting -- and it was twice the hassle.
Use a pair of draftsman's dividers to measure the run of brass which you need. Find logical end points - bulkheads, ladder spaces, gun tubs, etc. Transfer the measurement to the brass and cut (measure twice - cut once). Work in shorter lengths 2 to 3 inches tops. Resist the urge to attempt to put the railings on an entire side of a ship, just because you have a piece of brass long enough. That is a sure recipe for a wavy railing.
Use a new #10 Xacto blade (that is the curved one) and cut on a piece of glass or bathroom tile. Cut with a rocking motion, like a paper trimmer. Use a fine sanding stick to remove any remaining nubbins. Go back later after the brass is attached and hand touch up any cracks or nicks in the paint.
Use a white glue to tack the rails in place. My favorite is Aleene's Tacky Glue from the craft store. Put a pin-point drop of glue at each end and every 1/2 inch or so. The white glue will grab, yet allow the railing to be adjusted. Once the white glue is dry - go back and run a bead of CA along the bottom rail to firmly cement the rail in place.
After you make your hand touch ups, overspray everything with a flat coat to kill the shine of the CA and to blend everything together.