Here are some exact examples of what you can do to set a song together.
You can get inspired by a number of methods such as:
1. A chord progression 2. A cool riff 3. A lead melody
One of things you require to do is generate a part that compliments your original idea such as adding a melody to a chord progression or adding up a chord progression to a melody.
Say you have a chord progression you truly like.
To make your musical painting, you have to know theoretically what key you're in. If you're weak on modal theory, but familiar with a scale or neck pattern that sounds fine against your chord progression, employ what you know to create a melody over it.
The easiest way to accomplish this is to find a multitrack recorder and record your chord progression on one track. Play it over and over a lot of times.
When you play it back, work out your melody on another track. As you're working on a melody line, try to take note for cool harmony among your melody and chord progression. If only one exacting note sounds totally great against one of chords, go with it and start building around that great relationship.