I've found that learning how to use the "Add Maneuver" node helps significantly in setting up and executing changes in orbital inclination.

First, find what they call the "Ascending node" or "Descending node" in your orbit. For a circular orbit, these will be the half way points between the apogee and perigee, I seem to recall. If you have a "set target" then the ascending/descending nodes appear automatically on the screen as green markers.

Add a maneuver marker on either the ascending or descending node point, then drag the purple handles on the maneuver marker to line up the orbit plane the way you want it. It'll give you a readout of the length of the burn required, next to your navball. Then you just point at the new target marker on your navball (dark blue target symbol) and execute the burn. This target marker will be at a point on the horizon "sideways" from your direction of travel, the maneuver node is what helps you remember which side. Remember to delete the maneuver node after you're done with the burn. More details here.

If you've got an ion-powered space probe with low thrust, you might not be able to get the job done with one burn. Inclination changes are most efficient at the ascending and descending nodes, and if you spend a lot of time burning at points away from those nodes, your orbit will do other things you didn't intend. So you might need to burn a bit, then wait till you come round to the next node, then burn some more, etc.