Fold the rag in with the spare and strap it under your saddle with an old toe strap.
Don't forget to wrap it in an old plastic ice-cream advertising pennant nicked from outside a corner shop.
Or, being a law-abiding young man (and all the ice-cream adverts locally had been nicked by marauding scousers), I used an old washing-up liquid bottle with the top and bottom cut off and the tub stuffed down the resulting 'tube'.
The Dunlop cement used BITD tended to stay sticky for longer so was adequate to get you home when using a pre-used tub as a spare.
I've got mine wrapped in a '82 Milk Race course marker that someone left at the side of the road for some reason.
Bit silly to do that, makes for a good wrapper. I'm too honest to go stealing ice cream pennants.