How can I disassemble and install an old Schlage door knob?

A pin in the small hole starts it. Pull on the handle as you push the pin in the hole. An ice pick or small nail will be strong enough to do what you need. After the knob is off, you may be able to remove the rosettes as you suggest, but a lot of the rosettes are removed by prying of the rosette by setting a flat screwdriver tip in the slot that is visible at the edge of the rosette where it meets the door. This is usually located on the low side of the rosette. After that is off, the 2 screws that hold the rest of the knob is visible.

This is for the other hole, this is just for the idea, the pin on this one is too big to fit in the hole of the rosette.

It takes a little finess to remove, first be certain you are able to depress the release. On practically every schlage I worked on, only one side will release. Once you know you have something depressed, lightly push and pull on the knob, you should be able to observe the slight movement of the knob at the hole were the pin is inserted, the hole will move one way then another to touch the pin. Once that is confirmed, pull the knob and release the pin after a firm pull is made. If it does not work on that side, try the other side. It will work.

So I have the same lock, I bend a fork to use as the push pin, wiggle it with some force and got it to come off.

You have to turn the inner handle until you see a small depression of metal. That is what you push into and pull the handle.