First, cut the baguette into 3 slices lengthwise.
If you have a technique for this that you prefer, use it. Here is what I like to do:
Over your cutting board, hold the bread vertically. (So the baguette is perpendicular to the cutting board.) Take your knife, and visualize where you need to slice to make three, vertical bread slices out of the baguette. From the front face of the bread, measure a third in, and make your first slice. Put that slice aside. With the remaining two-thirds of the baguette, now make a vertical slice right down the middle. And you are done! You now have three slices of bread. We won’t use the middle piece for the sandwich, so you can save it for a later snack or to make breadcrumbs, croutons, etc.