Most dogs like to lick or touch our faces. This one usually doesn’t have to have a treat as a reward but some dogs work better for wages. Lean down to your dog, when he licks your cheek or touches your face, say “Give me a kiss”. Praise. Keep doing that. It’s all there is to it. You’ve just put the kiss on a cue.
If you don’t like to be kissed in the face, teach your dog to kiss the back of your hand. You might even add, “What a sweet gentleman” or lady. If your dog is an enthusiastic kisser, you might put a stop word on it. Usually just standing back upright stops the kiss.
When training a new trick, always stop training that particular trick when your dog does it perfectly. This tells him he was right. Praise heartily and leave that trick to tomorrow. Make it fun, try a few times and then do something else until tomorrow.