Lucky I had a few practice runs, look at the state of these grommets. I would have been destroyed if this had happened on the bag!
Once I found this online tutorial I was away laughing. The grommet set came with most of the tools needed - the grommet hole punch and the pounding tools. I had my own hammer, mat and scissors.
The key point is to reinforce your fabric. On my practice piece the grommet is turning in its hole despite being fully closed. On the bag I have folded the fabric so there are three layers plus I inserted a thick piece of iron on interfacing. My grommets are very secure.
Second most useful tip was to punch the hole then cut out the fabric hole with the tiny scissors. Trust me it does not work otherwise.
Lastly I recommend rotating the pounding tool a quarter turn after each hit with the hammer for even closure. I read that on another tutorial which I can't find now.