This is how I paint the bases of my Ork, Death Korp of krieg and many other factions. The Desert Sand section is based strongly on Quarter Paint's "How to Make: A Desert Base" video on YouTube. So, be sure to check that out if you like this guide.

Desert Sand

1. Basecost with Zandri Dust .
2. Wash with Agrax Earthshade .
3. Re-establish the base colour with a drybrush of Zandri Dust again.
4. Drybrush slightly lighter with Ushabti Bone .
5. Light drybrush with Screaming Skull .
6. Wash rocks with Reikland Fleshshade .
7. Pick out occasional grains of sand and the points of the rocks with dots of Pallid Wych Flesh .
8. Finish with a base rim of Steel Legion Drab .

Cacti/Cactuses

1. Basecoat with Loren Forest .
2. Wash with Coelia Greenshade .
3. Drybrush with Straken Green , to bring out the raised details.
4. Lightly drybrush from a downward 45 degree angle with a mix of Straken Green and Ice Yellow (or Dorn Yellow ).
5. Add very small/specular highlights with pure Ice Yellow .

Light Sand (Bonus)

The method used for this base is largely the same as the above method, but with a different wash. I think the result looks noticeably different though.

1. Basecost with Zandri Dust .
2. Drybrush with Ushabti Bone , pretty heavily. I used a soft makeup brush in circular motions to get a smooth result.
3. Wash with Seraphim Sepia , pulling pigment towards recesses and away from the raised portions.
4. Re-establish the Ushabti Bone with a slightly less aggressive drybrush.
5. Light drybrush with Screaming Skull .
6. Add some White to your Screaming Skull and picked out a few small/specular highlights.