The straws are in space—I mean, in place—so now it’s time to tuck them into each other. The template is here to help once again: the thick gray elbows draw our desired weaving, showing not only which straws connect where but also which straws are above/below in the weaving. Here’s an overhead view of the result:
And here’s the play by play: stick one of the short straw ends into the slot waiting for it on the other side of its elbow, making sure to insert it all the way to the flexy bits:
Repeat for the other two straws, carefully referencing the template as you go:
Be warned: if you don’t get the weaving quite right (e.g. the straws aren’t paired up correctly, or the over/under pattern is backwards), the scaffold may get upset, and this petulance may be expressed by straining or ripping. This is fixable! Just apologize to the scaffold in a soothing tone, undo the straws without sudden movements, and try again. The scaffold may also rip even when you are weaving correctly, and this too is normal. Just double check that they match the picture and keep going.
Repeat for the other three corners, and the tetrahedron is finished!