Pronounce the sound "tu." When you say it, you will notice the tip of the tongue hits the roof of your mouth, right behind your teeth. The sound "tu" is used in a single-tongue technique.
Pronounce the sound "ku." Notice how it pushes air forward as the tongue pulls back, while the tip of the tongue touches the base of the mouth. The "ku" sound allows you to articulate a note by forcefully pushing a burst of air into the instrument. This sound is used in the second articulation.
Practice playing quarter notes on your French horn, and articulate each note using the "ku" sound. Continue to practice quarter notes using major scales, at 60 beats per minute. Aim for a clean articulation, and try to match the sound achieved through single tonguing.
Play major scales again on your French horn, but this time alternate between the syllables "tu" and "ku." Use "tu" on the first articulation and "ku" on the second. Continue practicing this technique until it becomes comfortable. Play each scale using eighth notes at 60 beats per minute. Gradually increase the tempo by adding 4 beats per minute, per day, until you can double tongue at 120 beats per minute.