One of the earliest you can Master is the Flashing Air Kick which you can use with the Kung Fu Shoes. You can find a Journal telling you how to do it after the second save room on the Galleon Minerva but here’s the action necessary: Down, Down-Right, Right +
(Down, Down-Left, Left +
if facing left).
You can perform this three times consecutively for a powerful finishing strike on the third. Downside is that it costs MP so make sure you have enough for how many kicks you want to perform.
Perform this on any enemy you see, a bonus is that during the animation of the kick Miriam is invincible so you could stand in front of an attacking enemy and proceed with the combo without being interrupted. Additionally it can be used in the air, which will also enable you to extend an overall jump (you can use this to get the “Plunderer’s Ring” early in the second deck area of the map by chaining the kicks).
To check progress on the Mastery of a Technique, open the Pause menu and go Archives > Techniques. When it’s yellow you’re all good for that technique and you’ll also get this achievement.