How to use large dildos?

While I've had a few larger than average toys in my toy box over the last year, I recently bought a real monster. A 15 inch dildo with a 3 inch diameter. It didn't sound that large...then I saw it! O_o
I managed to just about insert it all they way, but I was very uncomfortable for quiet a while afterwards.
And I'm too afraid to try again.
I'd love to master this new toy. But how can I get used to it? Previous toys haven't stretched me much. Can anyone recommend anything I can use to work myself up to using it again? Any toys you can recommend?

lots of lubes and graduation
ive found some of the larger but plugs can be good for working up and they stay in place to alloe you to stimulate the clit (more arousal=more elesticity down there)