Here are some the ATCs that I made and swapped out on Swap-bot.
I can't find the other scan of the other ones I swapped out so that will have to wait for later. But anyway here are three of them.
All three began with hand painted backgrounds, acrylic paint on recycled cardboard (product containers like pasta boxes).
I found the transfers don't work well when adhering to watercolor paper.
Note that the paint swipes on the background card lent a textured effect especially on the second card. The texture was not from the gel medium, it was from the raised paint parts being what the gel medium stuck to.
This technique takes only seconds to do. I swipe the image's paper with a thin layer, then adhere it to the background. I rub and let it dry for about 10-15 seconds then I removed the paper gently.
There is an art to learning how much gel medium to put on and how long to let it dry or to rub it. It can't be told to you. You have to experiment with it and find what works for you. Also the type of paper you are using matters. For example it is faster and easier to do a transfer from newsprint type paper than from a thicker paper source.
If you pull it off too quickly the whole thing can be ruined.
If you pull it off too late, it leaves paper residue on the background and you can gently rub it off with a dampened finger. This does not always work well, though, as too much rubbing can ruin the image.
I had many errors and mistakes that resulted in about a 50% rate of success. So in that way a project can take longer to finish or sometimes I have many ideas to try, some of which don't come to fruition due to the fact that I'm using original source images not photocopies.
It is fun to play with transfers but it is not always foolproof and mistakes are probably inevitable.