I bore out the side frame journals with a series of reamers until I get to few thousandths under 3/16" and then I press the bearings into the side frame. One thing that is critical, the shoulder your axles needs to be 3/16" diameter. If the shoulder is smaller you'll need a different bearing. Same thing if you axle shoulders are larger.
You can use twist drills but they tend to grab the material you're boring out.
Jay