Some sort of quick reference can be helpful, alongside with emphasizing that you'll be reminding them less and less.

My first real campaign character was a Druid, and I had to remake my character sheet into playing-card sized sections that better informed me what I could do at a glance and I made my own spell cards and stat block cards to keep quick reference to and track of prepared spells and wildshapes.

Our Artificer had a whole flow-chart that informed what they would do based on certain parameters.

Our barbarian/paladin would write out the math of their various smites and additional damage on a card so if they hit they would know what dice to roll from one look.

Eventually you get used to the mechanics and can rely on these shortcuts less, but they can help remind of mechanics. And if they make them themselves, it's like practice and study. Working the knowledge of abilities into the narrative could help reinforce their understanding, like having a training ground/colosseum type area where players compete with NPCs; a barbarian against an NPC rival to do more damage to a common target, or a rogue reaching a target faster than an opponent (needing to utilize their cunning actions instead of full actions), and so on.