I made a quick photo tutorial. I'm a visual learner myself, so stuff like this really helps.
1) Cut your yarn and place it together. Make sure its as close together as you can get it so you don't end up with gaps in your hair.
2) Cut a strip of fusible interfacing. Make it so its the same length as your, um, hair grid(?). Place it fusible side down onto the yarn.
OK, I admit I wanted to show off my cute iron.
Make sure the interfacing is fused onto the yarn
4) Sew. I like sewing interfacing down so I can see what the yarn is doing as I'm sewing, but it may be better for some machines if you sew interfacing side up.
5) Tear off interfacing and attach hair to poppet
I hope this helps somebody.
littlezo- interfacing helps make fabric more sturdy. Its used a lot in purses and stuff.