If you have the Alignment tool, there is an attachment that bolts through it to help you. (if you have the housing off the boat, A bearing press works like magic!)
There are also several home grown tools out there. Just recently I read of someone using a piece of pvc pipe to drive one in. DonS has posted the tool dimensions in the technical sticky at the top of the page as well. Look for "backlash and gimbal tool" or something like that.
If the gimbal housing is still on the boat you will need to use one of the special tools. Either a Merc genuine (ouch$$$) or make one from the plans on this site, up in the sticky section.
The easiest answer to that sounds a bit 'twee', but is the 'right' answer... It goes in until it stops. As you're hitting the drift to drive it in you'll heard the different sound. That means it's stopped. Visually, it's just below the lip of the flange the bellows sits on. That's as close as I can get. If it's not far enough, the drive will not go all the way on.
