I've been using GIMP for years, so its mostly learning by doing.
They have a big documentation here, where the whole program is explained, but I would recommend to Just open your image, look at the tool bar on the left, and test out what the different tools are doing. If you have no idea how one tool works look here, where the different tools are explained.
Here HQ I posted some time ago.
Use This website If you're using smartphone, open site in Desktop mode,easy to use.
Steps: 1. Copy img url from my post. Then paste it in load url option on home page of pixlr.
In interface, Select crop tool from left hand side icons.
In crop, select aspect. Enter width as 9 & height as 20 (1+ ratio).
Adjust image, hit save, jpg/png as you wish. (If jpg, use 100% quality)
There you go! Use for any image you want. Site has great and easy to use tools. And it's all online, so use it anywhere, anytime.
If you need anything else, tell me. Will be happy to help.
Have a nice day!