My suggestion is you be in a costume, in a nurses uniform or a doctors uniform. You want to look like an authority figure. You should order him to strip completely. He should be naked and vulnerable. He is to submit to your commands and follow your instructions. Do whatever examination you wish. If he is not use to taking enemas, I would suggest you not use more than about two quarts, since neither you nor he knows what his capacity is. Also test the water on the inside of your wrist to make sure it is warm, but neither too hot nor too cold.
You may or may not wish to use soap: use a standard, unscented, bar of Ivory Soap. Let the water run over the Ivory bar to get soapy water. Again. not too soapy, because you don't know what he can tolerate.
You should arrange things so that you are higher than he is. He is on the floor or in the bottom of the tub and you are standing tall over him. At the very least he should be laying on a bed. You want to be higher than him and in a dominant position. You want him to surrender to you in a submissive position. (If you want to be extra naughty here, have your nurses uniform be a dress or skirt type. Wear garter-belted stockings underneath but no panties. Let him know you're not wearing panties! This thought alone will drive many men batty! Make a point to stand over him so he can look up your skirt, but order him not to! Make sure he gets a good peak anyway!)
If you are using a standard drug store type enema & hot water bottle set then use the vaginal douche nozzle since it will be more comfortable for him. (The standard, short enema nozzles just don't feel very nice.) If you are planning to use a bulb syringe, then I suggest you use a vaginal douche type bulb syringe. This will cause him more erotic anal sensations. (And being "fucked" by a syringe designed for a woman will surely make an erotic impression on him! LOL!) IMHO: An open top douche bag is preferable to the closed top type, because it can be refilled more easily. You might have a two quart water jug on the side to fill and refill the douche bag.
Insist that he hold the enema for a period of time. Five on ten minutes. Long enough to enforce that he is uncomfortable but you are in control, even of his private body functions! (Again, you don't know how long he can hold it, so don't make it too long!) (Alternately, you might keep some sort of spanker handy and threaten to spank him if he leaks!)
At the end you should either have him masturbate himself to orgasm at your command, or you should masturbate him to orgasm. Having an orgasm will be his reward for being a good boy and taking & holding his enema. (It will also be positive reinforcement to teach him that good things happen when he is a good boy who pleases you!)
Then allow him to go to the toilette and release.
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For an alternate ending. . .
If you started out with him naked, tied up and helpless... It is important that he BE tied up and helpless for this routine!
While he is holding his enema, at your command, talk about how hot and excited you are by all this! Hike up your skirt (remember, you are not wearing panties), or do an erotic strip tease for him, then masturbate yourself to orgasm while he watches! Have a vibrator handy to pleasure yourself! Make sure to Oooh! An Aaaah! And Gasp! Put on a good show for him! This is no time to be shy! After you've had a good orgasm, you should redress and return to chaste dominance. Then masturbate him to orgasm, in a very business like manner. Then allow him to go potty!
When he is done, permit him to perform oral sex on you until you have another orgasm!
You will have fun and make a memorable impression on him that he will want to repeat!
Guzzle Guzzle Kid!
PS: Shucks, I didn't notice the original post was almost 10 months old. How'd it go? I still think my suggestions might be fun! π