Hello
Double tonguing is hard to explain in words but i shall try

Some of the experts on here will be able to answer this more clearly than me .
For normal tonguing you sort of pronounce "tu" to tongue . Wheras for double tonguing you have to pronounce "tuku" or "tuka" . Practice double tonguing on just one note at first, say a middle D and keep the airstream the same ... basically keep it legato and not staccato . Once you have mastered that ,play scales using double tonguing and this should help coordinate your tonguing and fingers .
Sorry , I've not explained very well . Hopefully someone else will help explain it more fully .
Julie x