The multiclass system becomes available once you hit level 2, and youll see the Add Class button right there during level-up. Since there's a level 12 cap, timing your class combinations is super important - you'll want to watch those breakpoints carefully. The real trick is planning when to grab specific abilities since certain features dont stack like Extra Attack or Unarmored Defense, and you need to time your power spikes just right.

For spellcasting builds, your spell slots pool together based on your total caster level but each class uses its own spellcasting stat which is pretty crucial to remember when picking combinations. Maybe interesting: dipping into Fighter for Action Surge is really strong since it works with basically everything, but you need to grab it exactly at Fighter 2 for maximum effectiveness. Watch out for anti-synergy stuff like mixing Barbarian with casters, since Rage blocks all spellcasting and can really mess up your combat flow.

When you're planning the build, pay attention to those level 4/8/12 ability score improvements - missing these can hurt if you spread too thin. Third-caster subclasses like Arcane Trickster need special attention since they add spell slots differently, and you'll want to calculate those breakpoints precisely. Good thing is you can always respec through Withers if something isn't working out but try to plan ahead especially for those crucial ability score thresholds.