You've probably figured this out already, but the blueprints are those rolled up sheets of blue paper that allow you to learn new combos weapons or vehicles that can be built by combining two other weapons or vehicles together.
On your map, you can identify these by a blue gear-like icon. The map doesn't indicate the height, so if you're having difficulty finding one once you reach it, check up on the roof, or on another floor of the building you are currently in. Weapon blueprints are pretty easy to find when you're in the right area - they glow slightly, some light blue and others with a golden hue. The weapons you need to build from the blueprint are always at close hand as well. If you don't see many weapons (or objects) lying around, you're probably on the wrong floor.
Once you collect your 25th blueprint, the Achievement will unlock. The game gives you plenty of time to complete quests, so feel free to explore a little and find those ones nearby that are indicated on your map. I popped this just after completing Chapter 2 of Story mode without excessive exploration. You don't need to build the items, just obtain the blueprints.