Hi i want to write a letter W on the bag, as you can see it has a kind of perspective to it, so the letter should follow this perspective. I am using Adobe Illustrator.
I would do it by using warp perspective tool. It will rasterize your text effect (you can do it on smart object as well, preserving capability to edit it later). You can get to the tool by selecting edit -> warp perspective.
First you need to create bounding box around the object you want to warp, after you've defined the area, you need to switch mode to wrap, and you can begin to warp the perspective.
I've used all the text instead of a single letter, but the principle will stay the same.
This is what I got after a quick fit-in. I bet you can get it even more precise, and add some effects after (like a blend on the bag shadow to appear on text as well) to fit it even better.