Can someone tell me how to properly use a sliding T-Bevel? I'm trying to layout some trim and can't seem to get the angles correct.

This is really something you would need to be shown I would think.

I found this video: Solving Tough Miter Angles for Staircase Baseboards - Fine Homebuilding Video. Perhaps it will be sufficient to demonstrate the concepts you need.

Place you T-Bevel in or around the corner and tighten screw. If you don't have a protractor to measure the angle of the T-bevel, take it to you miter saw. Place one leg of the T-bevel against the fence. Adjust the miter of you saw until the blade slot aligns with the other leg of the T-bevel. On the saw 0 degrees miter is 90 degrees angle. Add or subtract the number of degrees you had to swing the saw. In other words if the saw is set at 10 degrees when blade slot aligns with the T-bevel the angle is either 100 degrees or 80 degrees, depending on which way you move the saw. Divide that angle by 2. This will be the angle of the cut. Because the miter on you saw is labeled 0 and not 90 you subtract that angle from 90 to get the correct miter setting on you saw. That is (100/2=50 angle) subtracted from 90 is 40 setting on the saw.