Creating a new model i think is not possible with Skin-maker, you need a 3d program for that. (Milk-shape, 3dsMax, Blender, Maya)
I am in the process of gathering old tutorials that are almost lost from the internet entirely, some tutorials you can find here https://www.ut99maps.net/ if you scroll down enough under "Mapping docs & links" .
This site has a tutorial section that you can download but i am not sure if it has a modeling/character section http://ut-files.com/
Dont know if sharing this links works because its from web archive (page 1) https://web.archive.org/web/20011122004 ... ut01.shtml if not then insert the link http://www.planetquake.com/polycount/re ... ut01.shtml in archive yourself.
I have downloaded some webpages completely that i may share in a new topic myself but i am not ready yet.
i don't know about monsters but for custom characters it seems to be the easiest to use Milk-shape that has a build in skeleton for Unreal Tournament that you can use to rig your character (i am assuming this is for UT99 i don't know if its used for UT2004) if someone can confirm i would appreciate because i don't like spreading false information.
you then bind it to your new character that you import and made in Milk-shape or maybe Blender and you can then export it in psk/psa format i think.
psk is the model, and psa the animations, you can use default animations of another model/psa if i am correct but im not sure how this all works, I myself would also like a clear tutorial (i have not read the ones i found myself yet) .
Anyway, tools that are usefull are Milkshape, DN UnPSA Toolkit,SkelEdit, and psk/psa importer/exporter for your 3d modeling application (3dsMax has both import and exporter plugin/scripts. also Umodel if you want to export existing models to edit them yourself in another program. and 3ds2unr to convert your model in file format that UT99 loves.