I've done both - dissolved it in cool water then microwaved it, and dissolved it in cool water (not much - sort of made a paste) then added hot microwaved water to that.

I also add a little coconut oil, ACV, sometimes AVG, and a pinch of Epsom. (All are supposed to help your hair absorb and retain the protein.)

Because I add those things, I like my gelatin mixture good and hot! That way the additives cool it down a little, but not TOO much! And then it's ready to put in my hair after I co-wash or shampoo and detangle.
2c/3a/3b, Fine/med, Dense, high porous, & normal elas.

Lo-poo - Renpure Keratin, SMR, Giovanni SaS
clarifying - mix Johnson's H2T with low poo
Co-wash - Suave Nats. clar. or coco.
RO - GVPCB, Ion Eff. Care, TN.
LI - SMR, Cure Care, or RO
Seal - MO or Jojoba
DT - + honey, ACV, & EVOO to SM masque & GVPCB
PT - Gelatin!, CNPF
Styling - FSG, AIF, BRHG, LALS/PS, GF curl spray gel, SM Souffle/Smoothie, LOOB
Oils - MO!, EVOO, jojoba, GSO, SM Elixer, EVCO, VO5 Hairdressing