help with drilling out a hole?

[budsiskos]

i have a couple small plastic pulleys that currently have a 8mm hub hole which i need to make a 5/8 inch hub hole which is just under double the diameter. earlier today i tried to drill it out with a 5/8 drill bit with very poor results. could anyone offer me some advice on how to do this better? any clamping advice would be welcome too because its nearly impossible to keep the pulley from rotating as i drill into it. thanks