Kagehina:

I like to think they confessed during a fight, maybe Hinata

Bokuaka:

Bokuto just yelled "AKAASHI I-I LIKE YOU" randomly when they were hanging out 

Iwaoi:

Oikawa did a normal anime confession, ya know under the cherry blossom tree thing

Kuroken:

Kemma, he was sad because Kuroo was graduating and then Kemma confessed while crying

Daisuga:

They confessed during graduation, Daichi did it first 

Tsukiyama:

Someone was bullying Yams and Tuski got mad, then confessed after he scared the bullies away