If you just try and apply it normally, I think it uses a geoshell, which would explain why it interferes with the genital texture as there can only be one geoshell on the character. So instead, I went to the surface tab, clicked her arm (where it says "ARM" in the tab that is) then clicked the textures (there were two, I did both) where you click it, select a new texture and then you can click the option to use LIE (Layered Image Editor). Once it is loaded, you can load in a new texture to layer on top of the arm texture, you just need to navigate to where the arm tattoos are. I also added in the mask texture for this tattoo as well. I set it to MULTIPLY (very important) the tattoo texture.
I done this for both the base texture and I think it was called the translucency texture, if you look you'll see two spots that use the same arm texture, so I did it for both, otherwise if one doesnt have the tattoo, once blended in the render, it will change the colour of the tattoo.
After that, you can still see the edge of the tattoo texture on her arm, so I finely adjusted the Translucency setting I think it is called until it blended in with her skin.
Took me a while to figure this out as I am not that experienced in using the LIE, but I didn't want to do anything external, especially if I plan to share my DUF here as that would mean I would also need to share any edited textures. The thought DID cross my mind at one point as I could not get it to apply properly for a couple hours, but I like to do things in DAZ where possible.
Can't learn without trying and I am stubborn.