I created this cutting graphic to assist you understand how to properly slice up a tri tip because I know that many people have trouble doing so.
The most important rule for cutting a tri tip roast is to cut the meat against the grain. The meat will be more tender when cut against the grain as opposed to with the grain. The issue is that once the tri tip is cooked, the grain structure is difficult to see because it changes in the middle of the roast.
For instance, the tri tip in the image below has a lovely crust, but it can be difficult to identify the grain pattern unless you know what to look for.