This is my favorite method to do a provisional cast-on. You just need some waste yarn, your working yarn and your working needle. I recommend using a waste yarn that is slightly bigger than your working yarn so it will be easier to pick-up the stitches later.
Attach your working yarn and waste yarn with a simple knot and hold them as shown in the photos with the waste yarn on top and the working yarn at the bottom.
Place your needle between the 2 threads.
Go under the working yarn.
Go over the waste yarn.
Go underneath both yarns.
And finally go between the two threads again and then under the waste yarn.
You have now 2 stitches on your needle.
Repeat all steps to cast on 2 new stitches each time. When you have your number of stitches, turn your work and start knitting! Be careful about the last stitches cast-on, they are not really attached yet and can slip off the needle pretty easily if you're not careful.
Or if you're working in the round, join both ends being very careful not to twist your stitches and start knitting.
When you need to pick-up the provisionally cast-on stitches, you just need to follow the waste yarn with your needle and put all the stitches that are around it on your needle. Then you can remove the waste yarn and you are ready to graft your stitches or work a border... whatever your pattern calls for!