For a figure-8 bun
Have your hair loose and fairly untangled.
Gather your hair at the nape of your neck, as though to make a low ponytail.
Start twisting your hair.
Twist all the way to the end.
Start coiling it into a bun...
... but make sure that as you coil you put each coil under the last...
... until you have a long thin bun poking out backwards.
Now grab the top coil (the one that's furthest away from your head) and pull it up towards the top of your head.
Push your hairstick (I used this one) upwards through this top coil...
... then reverse it so it goes down under the bun, catching some of the hair lying flat against your scalp. I find that it holds more securely if you angle it so it pokes out through the bun, rather than parallel to the back of your head.
Complete! The tail you see sticking out at the side can easily be pushed under the bun, or you can leave it messy.
For an infinity bun
This is much the same, but sideways.
Hair gathered as for a ponytail again, but this time slightly off-centre. I tend to do mine to the left.
Twist to the end of the hair, as for the Figure-8 bun.
Coil as you would for a Figure-8 bun, but stay off-centre.
Bring the top coil over to the opposite side (the right in this case).
Push your hairstick to the right through the rightmost coil (the one that was furthest out before). Note I'm using a pen this time because my hairstick was too dark to see against my hair.
Reverse the hairstick (or pen, as the case may be) back under and through the bun, catching some of the hair lying flat against your scalp.
Complete (this time with the dark hairstick).
Hope it's helpful!