How could itt be your hobby if you don't know how to do it? Anywho, you need to an image editing program downloaded onto your pc/laptop(like Microsoft Paint or GIMP) then you save a blank render or screenshot, and open it with that program for editing. You can search the web for tips on how to use whichever program you have, and can also download them from the web. MS Paint is great for beginners, and GIMP is like a free Photoshop, for more experienced artists. The site also provides Sketcher, which you can find on the Send -> image page. I use MS Paint, but also use Sketcher for certain features.

Hope this helps!