Cut PE using a #10 Xacto blade on a piece of tile or glass. Cut with a rocking motion like a paper trimer. A #11 blade point will break more easily.
I generally paint my brass on the fret while I'm painting the model. Spray and then touch up nicks and cracks later.
Measure your length with a pair of draftsman's dividers. Measure between logical endpoints; bulkheads, gun tubs, ladderways, previous railing runs, etc. Resist the urge to do the entire side just because you have a piece of brass that long. Work in maximum 2 - 3 inch lengths.
Make your bends off the model. For a novice -- use square-nosed pliers or opposed razor blades. A fancy tool like a Hold-n-Fold or Etchmate is not necessary. Learn the mechanics then speed and improve your accuracy with the tool. Like all tools it takes parctice to perfect. Round bends can be made by wrapping the PE arounf a wooden dowel or drill shank.
I recommend a white glue such as Aleene's Tack Glue from the craft store to tack the parts in place.
Put a pin-point sized drop at each end and every 1/2 inch or so and place the part. The Tacky Glue grabs, yet allows time to reposition. The water-based glue cleans up with water.
After the glue is dry go back with some thin CA and run a bead of glue along the bottom to permenantly afix the rail in place.
Work top to bottom and inside to out so as to minimize disturbing previously installed items.
Hand brush any nicks or cracks. Then spray everything with a clear flat to kill any glue shine and level the appearance.
Hope these suggestions help. Write back if you have any more questions