When trimming a hollow core door, take less than an inch off each side to avoid cutting into the void beyond the frame. The veneer face on hollow core doors also tends to splinter easily. To prevent splintering, score the cut line with a razor knife before sawing off the door.
Once you’ve scored the door, clamp a straight edge to it at the proper distance from the door edge.
Now use a circular saw to make the cut, with the edge of the saw resting against the guide board.
Once you’ve made the cut, sand the bottom of the door, and finish it with either paint or polyurethane.