Wigs Tips for altering clip-on twintail wigs?

The way I fix clip-in twintails is like this:
1. Take a small section of hair where you want to clip on the twintail. Pull out the topmost layer of the hair out of your way, then tie the remainder into a tiny ponytail.
2. slightly push the elastic down along the hair shaft until you can split the section of hair above the elastic into two with your fingers.
3. flip the ponytail into the space you made one or more times, the more you flip it the longer section you'll have to secure the clip. Usually one or two is enough but you may have to experiment a little.
4. pull on the ends of the tiny ponytail to tighten the elastic hair band near the wig cap.

What you'll have created is a little section that looks like this ()()
the bunch of hair will give you stronger gripping point for the clip, it won't slip around as easily as tying a pony the normal way or trying to grip to loose hair.
The top section of hair you clipped out, you can use to help cover any spaces in the wig cap, you can tuck it under the ()(), or over and fixed in place with the clip depending on your needs.
You can also gently pull a few hairs in the ()() section if you need a wider or flatter area to grip onto. You don't need to force the clip to hold all of the hair in the section. it's just a nice taught and textured area to clip into.

If you still really want to sew the hair on then I think you have to remove the clip which you're right would alter the shape so you'd need to do some styling work to recreate that. I would try this first since you don't need to make any major alterations to do it.

 

No problem. Let me know if you having trouble with the instructions when you try. I'm a bad photographer but I can try to take some pictures if needed.