Paul... welcome aboard....
Planing across the grain should take the worst of the rough areas out of your boards, but that won't get them as smooth as they could be; to do that you need to plane with the grain. Easy way to figure which direction the grain's running is to retract the blade fully, gradually start advancing the blade till it's just starting to take a cut. If you're working in the wrong direction, the plane will skip and chatter across the board, hence the retract the blade; with the minimum of blade showing, any chatter you get will cause minimal damage to the surface. To remedy, simply plane in the opposite direction, when you're sure the direction's right, advance the blade a little more until you're taking proper shavings.
One note to be mindful of; grain direction can change in a board, particularly around any knots etc; it may be necessary to plane towards the centre from either end, or from the centre to either end. Make a few test passes along the length of the board to confirm that the direction is uniform along the length, and if not, mark the direction that gives the least chatter in an area.
Make sure your blade's good an sharp and take light passes until you're confident that you have the direction sussed.
Above all, take your time, don't panic if it doesn't turn out right at your first attempt, we can always offer more help if you're still having trouble.