FF top's perspective:
Training and patience. Your sphincter is just a ring of muscle, and like other muscles such as your hamstrings, the more frequently you actively stretch the muscle, the more flexability you'll have for energetic fun and games. ;P If looking to prepare yourself in lieu of a Master or regular top, daily play with toys of gradually progressing size will be a good start. Have a larger dildo or plug in places like the shower to remind you to put in the work every time it's seen. Sleeping with a plug in your hole is another way I've found that helps ff bottoms to gradually expand baseline elasticity. Any toys with which you train should always take some effort to work in. If not, it's unlikely doing much to increase max stretchability. For intermediate steps between toy sizes, you can slide in additional fingers or combine toys to maintain a tight fit while moving and rotating them to massage the sphincter.
As should be assumed, use PLENTY of lube and, when going bigger, do so in comfortable positions where you can RELAX, let the tension out of your whole body. I've found techniques like conscious breathing (triggers physiological relaxation response), poppers(similar end, but chemically stimulated), and in depth understanding of one's anatomy (scientific, not practical) to be helpful in getting there.
And, of course, a top who is patient and knows what he's doing, when you're ready to try it with another.
Good luck!