If you have the time, try my second book How to be a Roman Dominatrix.
But if you're in a rush, if they're nagging and wheedling and you sort of want to dominate them, but feel a little lost, here's the potted version:
Tell your partner that, before any of their submissive fantasies can make it onto the menu, you need to explore your dom side. Tell them that this means - in session - you'll be doing exactly what you want, there'll be no make-believe, no play, except the fiction of the power exchange. Warn them that they'll be tested.
To prepare, rewind and work out what vanilla things you enjoy might extend into being a dom. If you're flirt, perhaps you'd like to tease. If you're sometimes a bit controlling, great, this is you opportunity to control. If you enjoy being served, congratulations, you have a slave for the evening, or day, or weekend...
When you do have a session, of course you can't physically dominate your partner. However, you can completely dominate them by controlling all the action and making sure it revolves around you.
Have a whip to hand and be ready to use it to chivy them along. Assuming safety, hit them hard enough to get the response YOU want. The fact of the whipping may turn them on, but they'll still flinch and obey. The monkey self doesn't like getting hurt.
When they're not busy serving you, or being victimised, they should be waiting on you quietly, kneeling at your feet or standing at your shoulder, without squirming or complaining....
In fact, that last, is probably a good place to start.
Make them run the gauntlet of quiet submission before they get to the good stuff. "fetch my book. Pour my wine. Now kneel at my feet and wait." And make them wait, really wait, long enough to get bored, then turned on by the fact they're bored and can't do anything about it.
Eventually, once you have explored what you want from domming, then there's an opportunity for those of their fantasies that fit, as entries, interludes and finales--if they still want them :)
For spicing up your sex life with some time-limited FLR and Femdom, see my Femdom self-help guides....