how do you change a 306 barrel if you dont have the old key??

One would need to drill out a cam from the old barrel that could have been freed by the ign key.A 4mm drill bit is OK.

There is NO CAM on the 306 ignition barrel. There is however a small square shaped pin which retracts when the key is turned.

l assume u have already removed the plastic covers below the steering wheel?

Look directly beneath the ignition barrel. You will see a small square-shaped pin on the barrel. This pin protrudes 1/8 inch & holds the barrel firmly inside the black metal steering column. Only the key can cause this pin to retract.

BUT.. without the key, u can cut away only a small thin part of the black metal. OR, u can drill 1/8 inch of the pin is height away. l recommend the latter.
Do not forget to disconnect the 6-wire plastic connector, & draw the barrel outwards.
Do not be concerned if ur new barrel has different coloured wires from the original. But the 6-wire connector MUST be the same.