Set the tile on the floor to determine the best layout. Do this before you adhere it to the floor or add grout. This prevents you from having to cut many tiles in angles to allow them to fit the available space. The goal is to use as many full-size tiles as possible on the area. Try to lay out the tiles in such a manner that the ones along the perimeters are half size or bigger.
Stretch a tape measure at the longest length of the angle in the floor or wall area that requires a section of tile. For instance, if the area is a triangle shape, measure the longest section at the base of the triangle for the length. Jot down the length.
Measure again from the center point of the longest length to the longest depth in the space. Write down this measurement. Think of the angle section as a triangle and place the measuring tape down the center of the shape. This leads to a measurement from the center of the base -- the longest section -- to the pointed end of the triangle. Use the same technique on all the angles you must measure.
Cut the tile with a tile cutter according to your exact measurements.