Be prepared to tutor them in the rules.
Much like video games have "tutorial" levels that are basically an exercise in playing the game, you will need to set up situations and encounters that are simple and actually walk them through the rules a few times before they can be expected to master them.
Make them accountable for knowing their own characters.
Characters in D&D have many special powers, spells and magic items that all have unique powers and uses in as many different situations that all have to be reconciled with each other. As a DM, you are responsible for adjudicating how these things act in concert, but the players should be able to tell you exactly what their powers and abilities are and how the work.
If a player looks to you every time to explain what their options are, what their attacks are, how to calculate their attack roll, it shows that they don't really know their character well enough. In these case, eventually you must push back and make them figure it out on their own. Like spoiled children, if you do it for them every time, they will never do it on their own.
You do not have to be a jerk to accomplish this, though you will potentially have to call out players at the table.
GM: Ok, Warrior, it is your turn, what do you want to do?
Unprepared Warrior: I attack the nearest one... I rolled a 6.
GM: That is a miss...
Unprepared Warrior: Oh, wait I have to add my proficiency bonus...
GM: Ok, what is the total then?
Unprepared Warrior: Um, I don't know... *looks at you expectantly*
GM: Ok, we'll skip you while you figure it out. Moving on... Mage! It's your turn!
Guaranteed that after being skipped over a few times the unprepared player will write down their attack bonus!