(Edit: Title fixed. Goodness I feel silly for taking so long to figure out how...)
Originally this idea came from the "Hot glue lining a wig?" thread. I also did not want to put hot glue directly on my BJD's head because I actually like her bald sometimes and I didn't want to peal off the glue each time. I also really didn't want to put the glue on her wig either. Squishtaru's post in the thread mentioned making a hot glue wig cap which inspired me to grab my glue gun and get working.
So I started experimenting. I tested my hot glue on some different things like wax paper (of course it melted to the glue but the kid in me says try it once anyway), scotch tape, and masking tape. The scotch tape was really hard to peal it off of, but the masking tape worked really well.
So I tried wrapping my doll's head in saran wrap and putting the masking tape over it. (EDIT: This method of making the wig cap was originally suggested by Squishtaru in the above mentioned and linked thread) The glue got stuck in the little puckers of the masking tape and pulling it off was not fun. This method might work for some but I apparently don't have the patience for it. (I would love it if Squishtaru would make a tute for this method in the future if at all possible.)
So this is what I did instead.
I laid down masking tap in strips on some wax paper and began putting on glue. I recommend using thicker lines of glue then shown in these pictures unless the fit between your doll's head and wig is tight. My problem is the wig was big on her so I needed to actually use really thick lines of glue to make up for the space.
Once you start it pealing, it comes up really easy.
To make the circle, I just measured around her head with it, dabbed on some glue to one end, and then stuck it to the other end where it would be nice and snug on her head. Now is a good time to decide if you want the flat part of the glue (the side that was stuck to the tape) facing her head or the wig. I tried both ways and mixed both in these pictures. For the one my doll is currently wearing, I made it with the flat facing her head and then flipped it (as I will show later on) so the rounded size will eventually face in. Really this part is up to you to decide and experiment with.
Then I took another line of glue and put a bit of fresh glue on the end. Stuck it to the back of the loop. (You can do this while the ring is on your doll or off. The hot glue peals right off the doll the second it dries)
Then of course you measure and stick it to the other side.
Repeat in the other direction and don't forget to tack it in the middle where they overlap.
Now for the flipping part I mentioned before. I did this because it fit on her head better with the wig. (since the wig is so big)
(More in second post)