Way of the Samurai

All in-game unlockables are obtained when you've accumulated a set number of points. You score points every time you beat the game in amounts depending on your title. Note that higher difficulties multiply these amounts.

Basically, by playing the game and enjoying yourself.

If you want to rack up points as quickly as possible, you can go for the "Random Slasher" title, which requires you to kill at least 180 opponents. This is very easily done by taking on the 100-man challenge at the Foundry, on the morning of the first day. Complete it twice, then leave Rokkotsu Pass; instant points!

Oh, and though I'm not 100% sure about this, I think you'll obtain everything there is to unlock in the game once you've gotten 3,000 points.