If you don't have a vise.

This one from Harbor Freight has been excellent.

• Put a drop of Loctite 609 on the shaft tip
• Open vise and align shaft and pinion

• Crank vice closed till shaft touches vice jaws
• Open vise
• Find a suitable washer or nut to allow shaft to protrude from pinion

• Crank vice closed again

Voila!