Write out your key board and vocal (melody line) first. The first staff (this is where you will write your notes) will be for the vocal line. Use pencil and keep an eraser handy to make any changes as you go. The next two blank staff lines will be for your piano part. You can purchase blank piano/vocal sheet music paper for just this purpose, or you can simply draw the appropriate clef signs in to make your own paper. Writing piano and vocal parts down together gives you a basic lead sheet for your singer to practice.
Write out the instruments that have the largest parts first. This might be a rhythm guitar, the piano or a couple of violin parts. By notating the parts that are larger first, you will have a good reference for adding instruments that have small, intermittent parts or a simplistic rhythmic pattern. Instruments on the bass clef will often be written last, if you follow this pattern. Solos may be written last as well.
Play your parts as you write. An electronic keyboard that mimics a lot of instrument sounds is the ideal instrument for getting an idea how everything will sound. To make sure your parts sound good together, play and record your parts, then listen to your recorded part before you begin notating a new instrument parts. This allows you to work better with more efficiently with multiple instrument parts.