The first hinge pin for the door goes through the base of the cabinet and up into the edge of the door you rounded in the previous step (see photo). If possible, use a mini drill to drill a hole through the base of the cabinet and up into the lower edge of the door to at least start positioning your pin. If you drill the starting hole it may prevent the wood from splitting as you insert the pin. I use short dressmakers pins to hinge my doors, (the type that hold sleeves and collars on new shirts). You can also use fine brads or 'dollhouse' nails.
Before you fit your door over the pin, check that the pin is square with the edge of the cabinet and isn't bent. If the pin is bent, straighten it or discard it before you attempt to hinge your door. If the pin is at an angle coming up through the base of the cabinet into the door, it could push the door off square, or come out through the back of the door.