Everyone has their favorite tools when it comes to marking fabric. Pencils, pens, chalk, markers, tracing paper, wax... So many options, each with their own pros and cons.
My favorite is this two-sided marking pen. One side comes off with a damp cloth, the other disappears within 24-48 hours. It's a gamble, sometimes- I prefer to use the disappearing side (it's magic!) but I never know when my sewing will be interrupted and if I'll be able to return to it within 24 hours. I started using these when I used to do a lot of embroidery- I'd draw or trace my design with washable marker and then stitch over it. Since I already had them lying around when I started sewing, I stuck with them and they became habit. I also really like a tool my sewing teacher gave me-- It's a little white waxy chunk, and the lines disappear when you iron them. It's especially great for dark fabrics.
Here's a design I traced onto the pocket of a sling and then embroidered over:
Here is an article on different marking tools and methods!