Spend time practicing with an editing software program, there are many programs available, for beginners and experts.
For a list of free ones click here:
Free Editing Software
If you have a program, practice something that you have not tried before, here is an example from Nick McDonald
I’ve been trying to improve my editing skills by following tutorials in Practical Photography magazine, using images from my collection.
The idea here was to take two shots from the same viewpoint at different times of the day and then blend them into one image.
I had shots of the power station, taken several years ago, over the space of about an hour around sunset. I selected two and blended them in Photoshop by aligning the two images as separate layers and then applying a layer mask and then applying the gradient tool.
Photographer: Nick McDonald