>>565
If you are really starting at zero I'd suggest you watch a few basic tutorials for the use of automatic1111's Stable Diffusion local install. Get familiar with the txt2txt basics, inpainting and the use of loras /lycoris for a start.
You'll find tons of base models and loras/lycos ranging from photorealism to anime on civitai.com. No r-18g there other than some tame vorestuff and a wee bit of blood but plenty of usefull stuff nonetheless.

Most r-18g loras can either be found here or on pixiv. (just search for "r-18g lora") There are some hidden gems though that aren't tagged properly.

As far as I know there isn't any straight forward lora for skull- or eye- penetration etc. However there's a lora that might be of interest to you here: (scroll down to the "head destroy lora" entry) https://monster-maker.fanbox.cc/

I haven't gotten it to work for txt2img, but it works very well for inpainting. it basicly allows you to put holes into the head, and generate some nice brainsplatter. You should be abled to produce some nice entrance holes or even brain penetration...

Another hidden lora-gem can be found here: https://kemono.party/fanbox/user/24196387/post/5655457 (I just grabbed the link and then Kemono went down. If it still doesn't work in a day or so I'll upload it here) It's a dissection lora. Not the prettiest but so far the only one I know of.>>565