Excellent question.
You need one of these:
First, use the curved edge of a paint stick, or a 3 in 1 tool
to run down the roller scraping the paint back into the can.
Then, what I do to clean a roller is take an empty 5 gallon bucket, fill it with some water and spin the roller clean right in the water(usually run the roller under the tap for a bit to remove some of the heavy paint, otherwise you'll be re-filling the bucket for days.
Once its clean, rinse up under the tap and then spin the shit out of it in the bucket.
You should be good to go after that.