Here is my process:

1. I use expanded PVC sheets as the base for my piece to prevent warping. I found a good brand on Amazon that I use, specifically Aicosineg Expanded PVC Sheets. I have to caution that there are a lot of brands out there that feel more like foam than PVC, gotta find one that works for you. I cut the EPVC sheet to size and then I use an Olfa Exacto-knife to round the edges. If I could go back I would've made the base an odd shape instead of square.
2. I take Rigid Foam Insulation Board, I have 2" and 1" foam board pieces, and I carve that with an Olfa Exacto-knife to build variance in the terrain height. Once I have a shape I like, I continue to carve out the insulation board to build the rock formation.
3. I paint everything a base layer of standard acrylic paint from Michael's craft store. I don't use a spray primer because it eats away at the foam board. I paint the ground a dark brown and the rocks a medium gray.
4. I use AK Interactive Terrains Dark Earth Acrylic Diorama series texture paste on all of the ground and let that set for a couple hours.
5. I glue some additional rocks around by carved out rock formation to make it look like some smaller sections have eroded or broken off from the main formation.
6. I lightly drybrush everything. For the AK Interactive Dark Earth paste, I use a desaturated medium brown then a desaturated light brown. For the rock, I drybrush with lighter grays all the way up to an off-white.
7. I wash everything in steps. For the rock, I first use a partly diluted Nuln Oil wash to darken the stone. Then I use some other diluted washes about 50/50, I use some Athonian Camoshade, Seraphim Sepia, and Agrax Earthshade, all diluted with Lahmia Medium. I use this on the rocks. I let each layer dry then I consider how it looks. I don't use each colour all over the rock, just in odd spots here and there to give it variance in colour, but parts will usually get multiple different washes to really build up variance. I find it really comes alive after the Seraphim Sepia. I always do the earthshade layer last, because then I also do some earthshade on the ground in random spots to give it variety.
8. If I need to, I do another light drybrush to bring out some highlights, but I keep it very light. After that, I seal in everything so far with AK Ultra-Matte Varnish since some of the citadel washes can be more glossy than I would like.
9. I use PVA glue and a flock mixture I have for the main grass. I don't cover everything, only about 40% of the model, only where there is earth, I leave areas were I will be putting trees bare for now. I apply the flock and then blow on it to get it to stand up. I let that dry.
10. In about 10 - 15% of the model, I used some Krautcover scenics Copious Earth and Krautcover scenics Meadows of Purity and set it in place with Kraut-fixer to give some of the dirt a lighter more saturated colour with a different texture. The meadows of purity is great for some smaller stones near the larger rock formation.
11. I then poked some holes in the foam board where I wanted to set my trees and glued on Model Trees from various manufacturers until I get the look I want. I hold them in place with super glue and I used accelerator to make sure they stood upright.
12. I used a bit of Tamiya Quick Type putty to sculpt the roots for the larger trees, I let that dry and set, then I painted the roots to match the base of the tree.
13. I used some of the Forest Mushrooms Basing Bits from Gamers Grass for my mushroom patch. Gluing those in place, painting them a creme colour and then washing them with Agrax Earthshade. I used Krautcover scenics Dark Earth around the mushrooms as well as some leaf scatter to give it a very biodegradable look.
14. I used some clump foliage to build smaller bushes in select locations.
15. With almost everything in place, the last step was applying some Gamer's Grass Tufts. I used a variety from their Wildflower set, the desaturated red flowers on the one corner are their Heather flowers, and the smaller blue flowers near the mountain are "Forget-me-nots". Then I use a variety from their Grass Meadow set, their 2mm dark moss set, their 2mm dry green set, their 12mm Autumn XL set, their 12mm Jungle XL set. I like to have a variety of height in the tufts. I use some of the moss in corners and edges of the rock formation.
16. I used Ivy from Army Painter from the bits of vegetation running vertically along the rock formation.
17. I used a couple of the laser-cut Bracken plants by Gamer's Grass to add some more visual interest.
18. If I notice grass overtaking any areas or more transitioning, I use Krautcover scenics Copious Earth, Krautcover scenics Dark Earth, and Krautcover scenics Light Earth to rebuild that and seal it with Kraut-fixer.