Can I get some guidelines related to editing images with GIMP?
Its not easy to learn it without any detailed guidance.
How can I sharpen my gimping skills?
The gimp docs, youtube and various gimp tutorial websites already mentioned in the comments are invaluable. I have found that I made the most progress when I tackled photos that I wanted to edit. Whether it was an event you attended, a holiday, a topic, subject or chosen destination that you had in mind (instagram, etc), start with some photos and experiment with "getting the most" out of them. For each photo try google searches like "gimp portrait tutorial", or "gimp separate out background", or "gimp restore old", etc. Gradually over time you will build up your repertoire of what you can do, and be able to selectively apply the techniques you've learnt depending on the situation. After a while, you will be able to drop gimp from your searches, and even use other tutorials but translate them for yourself into roughly equivalent gimp edits. (Admittedly not always obvious or possible.)
Have said all the above, a lot of photo editing to me seems to revolve around the following topics, so I would concentrate learning how to do the following in gimp:
One could go on (and other people will have different starting points) but with the above I find I can do a lot of what I want to do. Experiment much, have fun, trust what you think looks good.