Hi Julia, The procedure is actually really simple!
If in Photoshop you use the same names of the slots that you have in Spine in combination with the skin tag, Spine will automatically add a new skin and add the images to the correct slots!
Here's the relevant user guide page: Import - Spine User Guide: Data
And here's the procedure in action (I'm importing a recolor of the owl project there) https://youtu.be/kU26B5465SQ?t=406
Let me know if you need any further clarifications, I'm sure you'll love it 😃 (I do love it)