This one is pretty damn simple. When you’re licking her clit, it’s completely natural to lick it just like you’re licking an ice cream or something. I.e. using the top part of your tongue.
The trouble is your tongue actually is slightly rough so it can get a grip on your food better. I mean it’s not crazy rough like a lion so you can lick the skin off a gazelle or anything, but it’s still a little rough… and a clit is very sensitive.
But the underside of your tongue is smooth as silk…
…I’ll wait while you all experiment with licking your lips or something…
Now ladies, imagine that sliding all over your clit….
Bonus Tip: It’s much easier to lick with the underside of your tongue when you’re in a 69 position.