Instead of monsters rolling vs your AC, you'll roll defense rolls vs their attack DC.

Your defense check is your AC minus 12 (for example if you have AC 15, your defense check is +3) and you can list it instead of your AC on your sheet.

For spells that do saves, instead of me rolling the monster's save, you'll roll spell attacks (your normal spell attack number) vs their DC. It's not the same as their AC, it depends on what save category (dex, con etc) it is.

Instead of me rolling for wandering monsters, I'll ask you to do "encounter checks". You can use insp and so on on these. Different dungeons use different values. (For my home-made dungeons I usually use 1-3 on a d20 means that a monster shows up.)