Cutting it the right way means slicing it against the grain. The grain of a tri tip runs radially from the corner opposite the crook in the meat. This means that cutting the piece in two from corner to crook and then slicing each piece it gives you the best results.
Take a look at the video below for a visual guide to cutting this delicious piece of beef. Technically you could start at the thinnest corner, cut perpendicular to the grain as you move along but it is important to note that you will have to adjust your knife angle a bit each time. After all, it is a quality piece of meat and not a vegetable we are cutting here. This is why we prefer you cut it up into two-chunks. You will find that it is easier to get slices that are perpendicular to the grain that way. Check out this quick, 30-second video below to make it easy on yourself.
For more on tri tip, see our post on smoking tri tip BBQ style.