You'll want to visit ota gyuichi in kyoto, south of honnoji temple to start the crest collection quest but make sure you've unlocked yasuke first since he's essential for this. The crests are scattered throughout various castles and fortresses, with many requiring yasuke's strength to move heavy obstacles blocking the way, so don't try accessing them early without him.

Most crests are tucked away in places like sakamoto castle and miki castle, where you'll need to solve some environmental puzzles and handle some combat along the way. The whole thing ties into historical japanese clan emblems, which is pretty interesting if you're into that sort of cultural background. Once you've gathered them all and return to gyuichi, you'll get some really nice rewards including the falcon's eye legendary yumi bow, a multishot engraving that lets you fire multiple arrows at once, and an oda kamon banner to display at your base. The 1,000 xp is a nice bonus too.

Tip: while exploring harima and wakasa for the crests, keep an eye out for spots where large crates or barriers seem suspicious - they usually hide a crest behind them and you'll definitely need yasuke's strength to clear the path.