Deciding on which direction the nap will lie on your garment is a fundamental first when working with velvet. The pile of the velvet will run smoothly in one direction and be more bristly in the other. Think of stroking a cat. Traditionally the pile runs down a garment and will show off the sheen on the velvet.However, if you prefer a more dense colour and a mat finish you can have the pile running up the garment. Theatre costumes will often have the pile running upwards to show off the colour under the stage lights. But in fashion it will usually run downwards as you run your hands over the garment. Either is correct, it just depends on what you prefer.
But once you have decided, make sure you have all the pattern pieces running in the same direction. It’s a good idea to mark on the reverse of the fabric which way the nap is running so you can be consistent.