Here's the method I use:

First, like has been mentioned, I'll feed them 2 mice at a time, one right after the other, for a few weeks or so. Sometimes I keep them on the 2 mice per feeding for a few months if they're slow growers.

When it's approaching feeding day again and I'm going to offer a rat for the first time I'll move one of the rat tubs near their cage and leave it. I'll move it near their cage maybe 3 days before they're going to be fed so they've been steadily catching the scent. I like to check on them a lot to see if they're roaming around. I don't offer the rat until I've noticed behavior that says to me they're ready to eat. When I do offer the rat they usually tag it immediately. I haven't had very many that have been stubborn about switching over, so maybe I've just been lucky, but I've been using this method for a little over 10 years and it's always worked. If it doesn't work the first time, keep doing it. The longest I've ever had to keep this up for was maybe a month.

I breed my own mice and rats, so it's obviously easier for me than it would be somebody who doesn't, but even if you just buy a rat a few days ahead of time and can keep it in a little cage near your snake's enclosure it should work the same.

If you use f/t, instead of thawing the rat out in water set in on paper tower and let it thaw out overnight near your snake and you should get a similar result.