There's plenty of tutorials you can try if you're having troubles! I'll link one on the FH forums and a couple of Youtube videos for you! I always learn best from actually watching someone do something.
https://feral-heart.com/index.php?option=com_content&view=article&id=6&Itemid=11 (https://feral-heart.com/index.php?option=com_content&view=article&id=6&Itemid=11)
https://www.youtube.com/watch?v=rAEgc3l2ByI (https://www.youtube.com/watch?v=rAEgc3l2ByI)
https://www.youtube.com/watch?v=cW06Rat6lq0 (https://www.youtube.com/watch?v=cW06Rat6lq0)
You're very welcome to PM me with any questions as well!
You could use Paint Tool SAI for the height map work, but you MUST use and open the height map in GIMP with the greyscale option and save it via GIMP as well, as the image NEEDS to be saved as a .PNG greyscale image.
Just basically open gimp and do the following:
File ▸ New ▸ Image Size (513x513px or 1025x1025px) ▸ Advanced Options ▸ Colour Space ▸ Greyscale. Open up your height map image that you made in Paint Tool Sai and copy then paste it into the new canvas I told you to make above, then save it over the previous height map you made. Just be sure there is only one layer in the GIMP re-upload, and no Alpha Channel. You are welcome to use the free Paint Tool SAI (http://www.mediafire.com/file/4gafr4e2u1vxbce/SAI.zip) put up for download.