There are a few ways to do provisional cast on so if one is
not working out definitely go for another!
One that is
quite easy to figure out is using the crochet provisional
cast on. Take waste yarn and crochet a chain longer than
the number of sts you need to start knitting. Then take
your working yarn and pick up (pick up and knit) the number
of sts required by inserting the needle through the little
"bump" on the side of the crochet chain. When it comes time
to take out the provisional cast on you can just unzip the
crochet chain and put the resulting live sts on your needle
and you are ready to go.
This is probably better explained
by someone else and I know there are several websites with
step by step pics if you just google provisional cast on