The way I done it on the Dolly tyres was ....

Put the rim in the tyre , cut a small slice in the tread where you want the tyre to be flat . I gently heated a bit of tin plate on the cooker ,trying not to get too hot or the tyre will stick !! gently press the trye down on the heated tin plate ,the slice will let the tyre open to create the bulge either side of the tyre ,but try not to have too much heat you dont want the tyre sticking or melting !! If you have any old wheels/tyres give it a go with one of them first ,but if you get it right it does create a tyre that looks like its been flat for a long time with the weight of the vehicle pressing in it 

Chris.