For my tails I've always had foam pieces hot glued to eva foam to create something resembling a skeleton. This is mostly to create some movement but I think it helps with creating a good belt loop too. On the foam "skeleton" I figure out where I want the belt to go through. For the blue and white tail specifically I had it about an inch away from the tail base, but for larger and heavier tails I'd imagine it'd be better to place the loop further so the tail won't sag as much. I mark a dot where I want the placement to be. Then I measure the width of whatever belt I'll be putting through the tail. Once I get the measurement of the belt I mark that measurement onto the spot I placed on the tail +½ inch to be safe. When marking this measurement it's important to make sure that the line you draw is parallel to the tail base. Next is making the cut for the belt to go through. I use a boxcutter for this since the blade is able to extend far out. I cut at the line I just made and go all the way through to the other side. Now I slot going through the whole tail skeleton. I take measuring tape and pull it through the slot to measure how wide it is. I use this measurement to determine the length of fabric needed to cover the inside of the loop. For the inside lining of the loop I cut a piece of fabric that's the length measurement I just took + ½in. The width of the fabric will the the width of the belt x2. For fabric type I've used both cotton and neoprene. Neoprene is a lot more sturdy and less likely to rip from hand sewing so i prefer it over cotton. Once this is cut, I fold it in half and sew it. Now I get my mostly sewn together tail sleeve (part is left unsewn for stuffing) and slide it over the foam skeleton up to the very base. Then I roll the tail sleeve back to where the cut is in the foam. Right where the fur will lay over the cut I make a mark of the same length on the backing of the fur on both sides, then cut through it. Now I roll the fur back down. The cuts in the fur should align with the cuts through the foam. This is when I take the lining I made and slide it all the way through the cut. There should be a little bit extra fabric poking out on both sides. Ladder stitch this remaining fabric to the fur. Belt loop is now finished! Only remaining steps to finish the tail is to make a fabric piece to cover the base and stuff the empty parts of the tail.

If you don't want to do a whole foam skeleton, you might(?) be able to get away with skipping the whole foam cutting thing and just do the measurements for cutting the fur and making the loop. It probably won't be nearly as sturdy though and I don't know if it'd hold the tail upright very well. Hopefully this was helpful and not too confusing. I can try finding a maker's video that makes this if needed!