Learn proper musical notation if you don't already know it. It's a good idea to keep a reference handy, unless you are extremely familiar with music notation. You can purchase an introduction to reading music at a music or book store. You can also refer to a free online guide (see References).
Use blank music paper to make a grand staff. Standard blank sheet music paper typically has ten graphs, each with five lines and four spaces. To make the grand staff, connect these ten music staffs together in groups of two by drawing a line between them to connect them. One line is for treble clef, the other is for bass clef. If your music store carries it, purchase blank staff paper already set up as a grand staff. Some music stores even carry music sheets with the clef signs printed, which can save you time. There is an excellent website that offers all of this blank music paper free of charge, including pre-printed grand staff paper (see Resources).
Sit down at your piano with your blank grand staff and play. Get a good feel for the song you're working on before you attempt to write it down. Work on a bar of music at a time. Hearing your chords and melody in the context of the entire bar helps you determine the overall rhythm and the duration of the notes and chords you play. When you have the first bar broken down into durations, begin by notating one hand at a time. Write down the music your right hand plays (typically on the treble clef) first, then add the notes on the bass clef. If the bass clef plays a more prominent role, notate the bass clef notes first.
Use repeat signs whenever possible. You can use repeat signs in music whenever one or more bars of music is played the same way over and over. Repeat signs will save you a lot of notation time and a lot of money on music paper.
Consider taking advantage of digital composition. The music notation software Finale allows you to create piano or other instrumental sheet music by dragging and dropping notes and musical symbols to a blank sheet music template. You can create professional printed sheet music for all of your songs using Finale. You might also want to use Noteflight, which is a web-based musical notation that works like Finale. Both of these methods allow you to listen to your songs while you compose (see Resources).