Once downloaded you need to unrar/unzip/whatever just get the files it. You should place a folder called "D3D9Gen b3" on your desktop with all the files from the archive in it.
Depending on your system:
-x32 systems use D3D9Gen (.jar)
-x64 systems use D3D9Gen_x64 (.jar)
Run (Double click) the .jar you need/want.
You will see a small window with 3 options:
-Set videocard by a list
---The list is pretty small, but if you are lucky you may find your videocard already. If so, skip to the
--- step "Once created".
-Set videocard by text
---You need to write down your videocard.
------ Nvidia GeForce 7900GS
------ Nvidia GeForce GTX 660
------ Nvidia GeForce 7800 SLI
------ Ati Radeon HD 2350 Series
------ Ati Radeon HD 2400
------ Radeon HD 6800 Series
---------- You need the exact name, the examples here and in the tools list should be enough to find
---------- the correct name. If you make a mistake you can redo the file, dont worry.
-Set videocard by text enables the option "Detect my card"
--- 3 modes are given: Fast, Medium, Slow
--- the modes will autodetect your videocard, the slower the more precise. This may not work if
--- if you have an "Intel HD 4000", or similiar, and a dedicated video card. This may only detect
--- the Intel card.
If you found your videocard, just click on "Generate DLL"
Once generated, you will be notified and the file will be placed in the folder, where the generator is. This should be your "D3D9Gen b3" folder on your desktop.
The generated d3d9.dll needs to be placed in your skyrim install folder, where tesv.exe is.
The default folder is:
If you placed the .dll in your skyrim folder, check the launcher. It should have your videocard rehistered correctly. You may need to delete "rendererinfo.txt" from documents/mygames/skyrim folder. This .txt will be recreated.
Your game should work fine now.