Buy a car telephone holder and use Google maps in your car. Simplest way. But as previous posts have said you don't have to go that far. If you're going from BTS Nana go into the far right hand lane at nana intersection so it seems like you're going on the wrong side of the road, but there is a lane there, and a bit further down you'll get to the tollway intersection on the right. From there just head north till you get to saraburi, then go on the ramp and head on Route 2 northeast towards korat.
Just after the Khao Yai intersection you'll find you can get on the new motorway which will deliver you to the outside of korat. No toll yet because they haven't built the toll booths.