Rules

You gain an archetype by selecting archetype feats instead of your normal feats. First, find the archetype that best fits your character concept. Then select that archetype's dedication feat, using one of your class feat choices. Once you've taken the dedication feat, you can select any feat from that archetype, as long as you meet its prerequisites. Most archetype feats are taken in place of class feats, and so these are called archetype class feats.

An archetype feat is subject to any restrictions on the class feat it replaces. For example, if you had an ability at 6th level that granted you a bonus class feat, but that class feat had to be 4th level or lower and have the dwarf trait, you could use that class feat to take an archetype class feat, but only one of 4th level or lower with the dwarf trait.

Source: Pathfinder Player Core pg. 215