While Super Buu & by extension Kid Buu are the most popular to give new forms to both within official media & fanart, I decided to focus on Innocent Buu.
My inspiration for this came from a sudden flood of what-if absorptions forms from the Dragon Ball SD manga where he cycles through the non-saiyan Z-Fighters quite fast.
Using that as a base, the SD style is the easiest. Buu just takes their facial features & any objects on their faces like glasses, and makes them his own. For me it was just drawing Goku's face, and then then rest, Innocent Buu. I used Dragon Ball Super's Goku face even though the context for Buu absorbing Goku would be the Buu saga when they first fight to give Trunks time to get the dragon radar. So one would think I would use a more fierce facial expression. I can see Buu having an jubilant reaction with Goku influencing him before ending Babidi's life.
The second design for this form is a fan manga known as Dragon Ball Multiverse. There was an instance where a version of Innocent Buu attempted to absorb Gohan but failed. We were later given a design for this form that inspired how a approached it myself.
I kept Innocent Buu's face, but giving him Goku's Gi & undershirt. I made them oversized, even for Buu to emphasized that they are being stretched on Buu's large frame, so they compensate by being a size too big even for him.
The hardest part in all of this was unironically the hands.