Honestly, it’s not an easy thing to pull off. An untrilled ‘r’ is supposed to be pronounced with the tongue in the same place as when pronouncing an ‘l’ - many people actually pronounce it wrong (this is a speech impediment), and while it’s not even noticeable in everyday speech (except in extreme circumstances, like Jonathan Ross), it makes the trilled ‘r’ much harder because your tongue isn’t in the right place. Having your tongue in the right place is half the battle. After that, just keep trying. Eventually, your mouth will simply get used to the movement.