If you look closely at a piece of tri-tip, you'll be able to see the fibers of the meat running in the same direction.. this is the grain. When you slice, the knife should be cutting across those to give a better, more tender bite. If you slice with the grain, the bite will tend to be more chewy.
Here's a top down image of our example tri-tip:
If you use a salt and pepper rub or even my Texas style rub on the meat, you'll still be able to see the grain when it's finished cooking and you can just cut accordingly however, if you use a darker rub or something that makes it hard to see the grain, then it'll be more difficult.
For this reason, it's sometimes a good idea to mark the grain or do something that will help you remember.
Insert a toothpick(s) indicating the grain direction just don't forget they are there and be sure to pull them out before you serve the meat.
Take a photo
Use the camera on your mobile phone to take a top-down picture of the tri-tip. After the meat is cooked, you can take a look at the photo to remind you of how the meat should be sliced.
I recommend separating the (2) sections of the tri-tip before slicing. Here's a diagram showing where the cut line is on our example tri-tip:
This cutline is similar on most tri-tips but not exactly.. this is why it's important to look at the tri-tip you are getting ready to cook and have an idea ahead of time on where it will need to be cut.
I had someone ask me if it was ok to separate these two pieces before cooking them and while I've never heard of anyone doing this, I don't think it would hurt anything or affect the quality of the cook.
Here's a diagram I made of our example tri-tip that shows the cut lines for each section:
You will notice that there is approximately a 45 degree difference in the slices between the section on the left and the section on the right.
Please note that it is not the end of the world if you just slice it all one way from one end to the other but separating the two sections and taking a little extra time with it will make it just a little bit better and is worth the trouble in my opinion.
Now that you know how to slice a tri-tip, go grab a couple and get some practice time in.
If you need a recipe, here's a few to start with: