Apparently this info is not in any easy to find place so here it is, so none of us ever has to type it out longhand again. 
Unreal skins and model packs come in 2 flavors, UMOD and Zip.
Umod you simply double click and the files get put where they go.
Zip is more common and takes a little tinsy bit of effor to get working.
Zip instructions: UT2kX
Models will likely have 3 files in the archive, skins only 2.
one ends in .UKX this is the actual mesh and animations for the model
Uncompress (just drag and drop usually works) this file to the folder called "Animations" in your UT2kx directory.
Another ends in .UTX These are the texture files used by the skin/model. Shaders, portraits, etc all go in .UTX archives.
So you uncompress them to the "Textures" directory in the UT2kx folder.
The last file ends in .UPL This is an system instruction file for unreal tournament to be able to find your skins/models, place them in the browser and fill in their pertainant information.
This goes in the "System" folder of your UT2kX directory.
Thats all there is to it. Start the game and the skin/model should show up in the model browser for you to select, you may have to look for them as they are arranged by race and not every skin/model is what you expect.
If it doesn't work:
Check to make sure that when you put the files in their directories that the unzip process didn't create an extra directory inside your existing one. This happens if people keep the relative path option on when zipping the archive.
Check that the UPL is actually in your system directory. If so check to be sure it has something in it, and or that it and the other files don't have the exact same name as another model/skin you already have.
If all else fails post around these boards somewhere, and someone should be able to help you.
UT99 is very similar, but a slightly different directory structure
ITs been a while since I did anything for ut99 so this may not be 100% accurate
there the PSA files go to animations, and psk go to meshes, or in the case of older vertex animated models they are .UNR and go in the animations folder.
The UTX and UPL files still go in Textures and System respectively.
Unreal skins and model packs come in 2 flavors, UMOD and Zip.
Umod you simply double click and the files get put where they go.
Zip is more common and takes a little tinsy bit of effor to get working.
Zip instructions: UT2kX
Models will likely have 3 files in the archive, skins only 2.
one ends in .UKX this is the actual mesh and animations for the model
Uncompress (just drag and drop usually works) this file to the folder called "Animations" in your UT2kx directory.
Another ends in .UTX These are the texture files used by the skin/model. Shaders, portraits, etc all go in .UTX archives.
So you uncompress them to the "Textures" directory in the UT2kx folder.
The last file ends in .UPL This is an system instruction file for unreal tournament to be able to find your skins/models, place them in the browser and fill in their pertainant information.
This goes in the "System" folder of your UT2kX directory.
Thats all there is to it. Start the game and the skin/model should show up in the model browser for you to select, you may have to look for them as they are arranged by race and not every skin/model is what you expect.
If it doesn't work:
Check to make sure that when you put the files in their directories that the unzip process didn't create an extra directory inside your existing one. This happens if people keep the relative path option on when zipping the archive.
Check that the UPL is actually in your system directory. If so check to be sure it has something in it, and or that it and the other files don't have the exact same name as another model/skin you already have.
If all else fails post around these boards somewhere, and someone should be able to help you.
UT99 is very similar, but a slightly different directory structure
ITs been a while since I did anything for ut99 so this may not be 100% accurate
there the PSA files go to animations, and psk go to meshes, or in the case of older vertex animated models they are .UNR and go in the animations folder.
The UTX and UPL files still go in Textures and System respectively.
