This is just like rubbing your belly button and sucking your thumb at the same time.
Below are ways to do it like a professional.
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Make sure you are really clean. Make sure you wash with an unscented soap or for a more intensive clean use an anal douche, which will ensure you're clean for any anal play that may happen afterward. Douches should only be used in little quantity though because they are very harsh on the body’s natural anal lining.
Rimming can be deeply pleasurable, as there are a whole bunch of nerve endings which surround your anus and many people find gentle stimulation with the tongue a real thing. However, if you or your partner aren't comfortable with such private licking, you could try a dental dam.
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The penis-having partner needs to be erect and stand with their legs slightly apart while their partner licks the anus while simultaneously reaching around to the penis to perform a hand job which is the trombone.