The “Practice Makes Perfect” achievement requires you to complete 55 trials.
From the main menu, select Singleplayer -> Trials mode. From here, choose any trial and completing each will count toward the total. There are a few different types of trials. There's a section dedicated to parrying attacks, defeating a CPU opponent with lowered health, and combos which make up the majority of trials.
There are 110 trials in all. You only need to complete 55 of them. Typically, trials #1-3 for each section/character are fairly easy, while #4-5 will much more difficult. Do as much as you can in each section/character and then move on to the next. There should be more than enough easy ones to get you up to 55.
Sometimes it helps to see what the combo looks like in order to perform it. VesperArcades YouTube channel has videos of the majority of trials, along with a top-down view of the players hands.
There's a glitch that allows you to stun your opponent in trial mode. When you stun your opponent, whatever you do afterwards counts towards the combo. Using this, you basically split all the combo trials into two parts. The stun glitch is detailed here.
Capcom plans to patch this glitch in the future. If you still want to use this glitch, you can remove the patch by clearing your cache, or decline to install the patch.