PART 1
Introduction to double tonguing. Practice with and without your flute.
Remember to play with a good sound even while articulating fast.
Use a metronome for EVEN articulation.
Try to equalize the tongue (T or K) and throat (K or G) strokes.
Play the exercise with:
2/4 TTT-KKK-TKTKT-KTKTK
4/4 TTTTTTT-KKKKKKK-TKTKTKTKTKTKT- KTKTKTKTKTKTK.
PART 2
Mix up consonants and vowels so you don't "trip over" yourself. Use combinations of T-K and D-G and Tuh Kuh vs, Tee Kee (Dee -gee), Too-Koo, Tah-Kah, etc.
I learned this trick from Alain Marion.
PART 3
Keep the air moving through the phrase while you play to maintain good sound quality and a musical line.
Be careful not to overdo the tongue motion while articulating fast. The movement is very very small. The jaw and embouchure should not be moving to produce the attack, just a tiny flutter of the tongue hitting behind the top teeth where they meet the roof of your mouth.
PART 4
Practice scales in many articulation patterns. I like Taffanel and Gaubert Daily Exercises #4. Use a metronome!
When playing two slurred notes I like to say Tee-Yah so I don't rush or swallow the second note.
It is important to practice quick articulations on changing notes so that you can carefully coordinate the tongue with the fingers and air.
Remember that the articulation is produced by SMALL motions of the tongue and not by a moving embouchure.

PART 5
Use the Allegro from JS Bach Sonata in C Major to practice double tonguing. First play without any tongue at all, using just air attacks, but don’t push the stomach muscles in and out for each attack. Then play double tongue on each note.
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More instructional videos on articulation.