Snap to grid

Could someone please refer me to a detailed description of how to get a sketch to snap to a grid. All I want to do is extrude a channel in a rectangular block but when I draw the orthogonal face of the channel I want to extrude, it gets placed in some infinitesimally small deviation from the grid line and the object surface. I have all the snap settings set to on but no matter how many times I try to drag the shape to the grid it never snaps to it. Sorry I am a beginner at this. I see a lot of people referring beginners to the tutorials but so far very few things seem I try to do work according to the tutorials I am watching. I am using an iPad Pro 12.9” 6th gen with an Apple Pencil. Also the software seems to constantly freeze and not respond. If I go to another program like procreate, it responds perfectly to my Apple Pencil.