How to roll/trill the letter R in a russian accent?
This manual looks pretty elaborate. Good luck!
(I can't do it, by the way...)
Russian r's are formed by flapping the tip of your tongue against the alveolar ridge (behind your upper teeth)--you only need to flap once or twice. A good way to learn how to do this is to say the word рука only give a really strong puff of air as you say the р. In your question you mention a Russian accent. That normally implies the way some other language is pronounced as a Russian might pronounce it, for example, as a Russian might pronounce English words (with a Russian accent). The trilling of the r in this case would be the same if you want to pronounce English words with a Russian accent.