This is the way I make packs which consist of a few steps
1. Getting the images/gifs
This can both be the most easiest and hardest part of making a pack, it boils down to how much content is available for the girl. Pornstars tend to be the easiest as they have to most variety (if they are well known ofc) whereas celebs tend to be harder. Anime stuff varies alot.
2. Tagging the pics/gifs
Most tags are very self explainatory, maid covers the girl in maid outfits or cleaning, waitress covers the girl serving drinks, behind a bar or food counter and so on. Try to tag as best as you can and remember the game only reads single tags. If you are not sure what to tag a pic you can either ask here in the forums/discord or try downloading a few packs from other pack makers and compare what pics they use for the tags.
3. Compression
I personal consider this essential as I like to have lots of girls so reducing the size of pics is the best way to go. For pics I try to round the lowest dimension to the nearest 1000 (e.g going from 3000x1500 to 1500x1000) as most people have 1080 monitors and the amount of space you save really adds up and the pic quality is usually untouched. I use Caesium to mass resize and compress. Solitude also made a guide which you can find
here that focuses on the more technical aspects of making a pack.
4. Making the girlsx/rgirlsx file(s)
All packs require a girlsx file and it will be unique. rgirls.x files are optional but I highly recommend making these, especially if the pack has nice variety. The stats of the girls are up to you, I would highly recommend again downloading a few packs from other pack makers and taking a look at the girls.x files and compare. I personally try to keep the stats on the slightly lower side as the game is pretty easy as is (it's more of a sandbox in some ways so having girls that need some experience before they become godlike kind of gives me a goal to work to). It's also nice to try to keep to traits and skills that make sense for the girl, e.g if the girl is a stripper then giving her traits like agile, flexible or fleet of foot keeps it in flavour (you could ofcourse also go the opposite way and have her a clumsy, slow learning bimbo if you wanted, in that case the player has to try to get to the better traits by using items or other buildings.).
5. Zip it up and upload it.
Hope this helps somewhat, if you want to know something more specific ask away here or on the discord.