[Fyron]

Create a new mod folder named DMM or DeathstalkersMountMod or Deathstalkers Mount Mod or something. Copy the Data folder for stock SE4 into this folder (including the data folder itself!). Then, extract the file from Deathie's zip file into this DMM\Data folder, overwriting the old one. Thus, the mod is installed.

You can also install it into other mods, but you need to be careful to make sure the mod is not depending on custom mounts that are in it already. And, make sure to back up the mod's file before installing DMM into it!