I can;t answer for sure, but considering many artists ask that you have the tattoos separate, and transparent. I assume they draw them separate and transfer them. Shifting and altering them a bit as needed

i find that this method works for me. also allows me to make changes to the tattoos if needed

Depends on what effect you want. I myself make the drawing separate then apply and transform and warp it to fit properly and use an overlay, soft light, or hard light layer depending on what effect you want. It's not really all that difficult.

I see, I just see so many complicated tattoos, that would make me crazy trying to do myself, since too much of a complicated design or pattern is hard for me (atm).

there are ways to learn. I have learned a lot of my own prowess from studying YouTube tutorials and the like. I don't consider things to be "too hard" just that it will take time to learn. The pay off is more than worth the time spent learning.

My only issue with tutorials on YT is the people hosting them, usually hides their tools/what settings they're using or make it so you can't see, like I wanna see where they're clicking on etc, but that's SAI though, because I wanna improve on my shading, and so many I seen there, they make it hard to see fully what they're doing, only showing the process of the art. Then again I could be looking wrong. I dislike the SAI/PS combo ones since I dont use PS, that's normally what i find when im looking for non-video type tutorials for SAI, so it can be work to find specifically what im looking for. Gonna try to find tutorial vids to improve on animal markings and linearts.

most tutorials I use, show my processes, tools, even downloads for brushes and other things. I don't use SAI for me; it's just not the appropriate program for what I need it for. I don't need a stabilizer as I prefer my little imperfect strokes for the organic look. If I need it crisp, I can always apple a sharpen filter. I namely use OS/Clip studio. But perhaps in your searches, maybe look up "best tools for SAI and things like that?

Yeah, i'll try that, thanks! :D