I measured only with a ruler how high the tile should be on each side, drew a line on the tile, and cut it. It turned out great. But the trick you refer to might be the following:
1. Hold the tile you want to cut (let's call it "A") on (as in "outside") the tile in the row above.
2. Measure exactly one "tile height" up from the floor (e.g., using a small scrap from a previous cut, or by another suitable method) and mark on tile A where this height lands. Do this along the entire width of the tile so you follow the exact contour of the floor.
3. Cut along the marking; Tile A will now fit perfectly into the hole.
Note, if you are to have a joint width between the tile and the floor, you should compensate for that and cut tile A slightly smaller (lower) than the marking (or use a "marking tool" that is slightly longer than a tile height).
Hope this is understandable?