First, confirm you are actually dealing with 90°.
I make a template using 1/4" MDF. A track saw works well for this. Make 2 templates one for each side, and dial them in either by additional cuts or a hand plane. When you're satisfied with the angles, use the templates with a pattern or flush trim bit.
Do this before laminating, that way if you screw up you haven't lost the whole thing.
If you're dealing with prepared countertops, leave extra length.
Last edited by DrRobert; 08-11-2020 at 12:01 PM.