I'm putting two push-pull pots into my guitar (volume and tone). On a normal pot you'd put the earth wire to the top of the pot but where can I put it on a push-pull?
You can just solder it onto the side part of the casing. Any clear are of the casing should be fine.. as most push-pull pots don't come with earth lugs.
You could shield your guitar and run EVERY earth to a ring terminal screwed into the shielding - you should really quieten your guitar by doing this too.