Predecessor Builds > Dekker > Dekker Magic Shred
I will update this as I go back to playing Dekker more. But this general build will get you great cooldown on your abilities.
Dekker has decent durability and basic attack, so early game you should focus on poking/zoning. If you scale well enough you should be a CC god in mid-late game. Marshall is crucial for this build. Sometimes I go for marshall right after time warp, to give my carry higher attack speed. Helping your carry outscale the opposite carry is your main job as support, and marhall is one of the best items to accomplish that
Don't be afraid to change riftwalkers for silentium or tranquility. Whatever you're more comfortable with, go with that
Prioritize your stasis bomb, then the containment. And for the love of god, stop stealing CS from your ADC