#tutorial sorta?

+ My first tip for doing hair defaults is learn how to clothes default first (lol, I told you this is a long answer...) just because I think it's a lot easier, familiarises you with SimPe and its a shorter project to build your confidence up. I used moogle's defaulting tutorial here which is just sososo good and I can't recommend enough. It's essentially the same as marja's but in think the order moogle did it in is a little easier. It's just a matter of preference because they're both super clear and have plenty of pics to help you follow.

You can use moogle's method to default hair as well but it doesn't tell you which textures you don't need to keep when doing hair defaults which can end up making the file a lot bigger for no reason, hence why I suggest marja's for hair defaulting x

+ Once you've got a hang of clothes defaulting, or if you just want to jump straight into hair - I'd suggest defaulting one of the later game packs (BV or later) and possibly doing one that's just for adults because it will be a smaller project. The base game hairs are a little more fiddly because of how young adult hairs were added later on. They also tend to have a bunch of hidden kid versions which makes it a little harder/time consuming/annoying to work out the ones you actually need to edit to make them show up.

+ To avoid wading through all those SimPe files however, you can just download someone else's version of the default and delete everything from the package apart from the 3IDR files and the property files. This will save you quite a few steps and makes sure you definitely have the right versions of the files. Just make sure you remember to edit the property files so they match the mesh you're using instead though.

+ Speaking of property sets, if you're defaulting a hat hair, make sure to change the category so it appears for all outfits (unless you're replacing with another hat I guess).

+ This is an issue just for the Apartment life hairstyles (i think?) but they're not 'familied' correctly, which means when a sim transitions in age, they don't retain their hairstyle. Copy one of the family strings and apply to each property set so they all match and this should mean your sims keep their hair styles. If it doesn't bother you, you can skip this step. For about 90% of hairstyles this isn't an issue though.

+ I don't think either tutorial mentions but if you're using a hair that says "grey linked to black", that means you will find the TXMT and TXTR files for elders contained within the black hair recolour as well, so make sure to extract those too.

+ General tip, and this is again preference, but I'd say always have a notebook with you. There are a lot of numbers to remember and I think writing them down is easier personally then writing them as a note on your computer. If you can colour code them while you're learning as well, similarly to how moogle did in their tutorial, I think that can be really helpful too.

I think that's all the wisdom I can dispense but please don't feel like you can't ask me specific questions either! I'll continue to add to this if anything else comes to me. Again, thank you anon for your question and I hope this helps x