When you buy the paint, buy a couple of plastic drop sheets. They only cost a couple dollars each and will stop paint drops on the carpet. We used tinned tomatoes to hold them down.

If you use powder in your washing machine, the boxes usually come with little plastic scoops. We save these, and you can use them to take the paint from the tin into the paint roller tray. Try to prevent drips on the edge of the tin, cos they make it messy when you are putting lid back on and moving tin later. If you hold brush under the plastic scoop while transferring from tin to tray, it will catch the drips.