Wigs Silkier yarn wefts?

I tend to use an old hair straightener for straightening my yarn wefts. I set it to a lower temp heat to straighten them—around 180-200F—and make a few passes through the straightener with each weft. That's what has generally worked best for me.

 

I also use an old hair straightener! I use a kind of hard bristled hair brush and brush as much out as i can before I straighten and it gets it pretty smooth!

 

I use actual silk. It's called silk roving, and is pre-spun silk, and since it is organic, you can dye it any color you like.

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The brand and material really make a difference--with natural fibers aside from silk, the length of the fiber is limited by nature, and the longer options tend to be more expensive as a result, since they spin up better and are sturdier.

Synthetics and semisynthetics (rayon, tencel, etc) are usually made as monofilaments and do not have a maximum length, theoretically. A lot of the more inexpensive yarns do have shorter lengths though.

One (destructive, so make sure you have permission or are using samples) way to check is to untwist your yarn of choice into a single ply and pinch from the tip and carefully pull out the fibers. If you keep pinching and collecting fibers for a couple of inches, you get a pretty good idea of what the average fiber length is going to be. If the lengths of your yarn chunks before brushing are much longer than the average length of the fiber, you're going to end up going bonkers trying to get enough wefts.

It really pays to be gentle and work in small sections to avoid distending or frizzing the fiber. You can definitely tame harassed fiber into submission with heat, but it's very tedious work and too much heat can exacerbate the problem instead.

Finally, take a look at your tools--most of them are sized for human hair and expect human sized sections, so there's often a gap somewhere when you hold them closed, and smaller sections of hair don't get pressed as well. Mine is like that, so I find it's helpful to pull the hair through with the plates positioned almost vertically, rather than horizontally. For really stubborn fiber, I will attach it to a bed post or similar and then both lead and follow the straightener with a fine tooth comb, so I can ensure the weft is in its flattest state possible and also apply tension to the fiber until it cools in place. It can take quite a few passes given how thin and fluffy doll-scale fibers are.

 

Carpal tunnel really does make everything more difficult

I'd definitely second what @Victoria Victrix said about silk roving as an option. If the material does not appeal, wool and acrylic roving are also reasonably available--though in some cases as roving yarn, which is fluffy and has less much twist-- and can also be dyed.

Good luck!