The only way to become proficient at classic knife cuts is to practice!

And keep practicing! 

If you can cut Batonnet (1/4" x 1/4" x 3"),  you can cut small dice (1/4" x 1/4" x 1/4").  All you have to do is make one more cut!

If you can cut Julienne (1/8" x 1/8" x 2"), you can cut Brunoise (1/8" x 1/8" x 1/8").  All you have to do is make one more cut!

Squaring off the item you are cutting makes your cuts that much easier.

Using a ruler or measuring tape is a good idea if you are not sure of a specific dimension.

You really need to practice Tourneing potatoes ... it takes lots and lots of practice to do it well!

If you practice you can make nice cuts every time.