First of all, welcome on the switch path! As always, take your journey at your own pace and don't forget to have fun!
I am afraid there is no guidebook on how to bring this up with your sub, you will know them a lot better than anyone here, but just as with any negotiation, try not to go in with too many expectations - they are your sub and may not be interested in switching at all, you need to be respectful of that and do not force it on them.
Perhaps approach the subject carefully, ask your sub if they have ever thought about the dominant side, and go from there. Inquire if they ever wondered how it feels like, how you in particular feel as their dominant, and gently nudge the conversation towards experimentation.
If you seek to be dominated by someone else entirely, that is an entirely different ball game. It could be helpful to start with a second dominant for your sub, teaming up to get to know each other as a group before taking the submissive role yourself.
Perhaps you do not want to include your sub in the journey at all, still you should be upfront about this and if so, how you approach it depends a lot on your sub, be gentle, respectful, and reassuring. It is going to suck and perhaps the best you can do is damage control.
I hope you two can find a way to make it work!