Buy your local OS map (assuming you’re not in the middle of a city – if you are, then maybe try a map of the area around your local trail centre) and go for a ride, trying to find different ROWs that you’ve never ridden before.
Really helps to build confidence in your navigation skills knowing that you’re not really that far out of the way if you get it all wrong.
I’ve made some great local loops* by just drawing an imaginary line on the map and heading out.
Oh, and if you’re new to reading maps and route planning – mark yourself a bit of string up to scale so you can get an accurate route length. It’s very easy to look at a route and underestimate – leaving you knackered in the middle of nowhere!
*OK I’ve also ended up wading through knee deep cowshit and shoulder high nettles too – but it’s all part of the adventure.