If you have a pH meter, you can do it in one of 2 ways.

1. Make a solution of Tris base that is a little more concentrated than 10 mM. Add HCl while measuring the pH until it is 8.0. Bring the volume to up with water to make the solution 10 mM in Tris. Note that pH is temperature-dependent, so the pH should be measured at the temperature at which the Tris solution will be used.

2. Make a 10 mM solution of Tris base and a 10 mM solution of Tris-HCl. Add one into the other until the pH is 8.0.

If you make a concentrated stock solution of Tris-HCl at pH 8.0 then dilute it, the pH will not be 8.0, so if it is important to you that the pH be exactly 8.0, you should make the 10 mM solution at that pH.