Hello, I was in your same situation pretty recently and it took a long time for me to find an approach to customizing that worked. Thankfully, I found something that worked for me and now it's my main way of enjoying the hobby. Here's my Siege Ironfist:
I don't know if the 3d-printed head would fit on Hound, but I replaced his face with a modified TR Shuffler's and it worked just fine.
He was painted with Rust-Oleum Painter's Touch spray cans, specifically Ivory, Wildflower Blue, Granite, and Metallic Warm Gold. I sealed him with Rust-Oleum's matte clearcoat, which is also a spray can. I had to spray these outdoors because of the smell, but you should bring the pieces into an indoor, ventilated area after spraying if it's excessively humid or outside the recommended 50-90 degrees. Additional detailing was done with a Molotow liquid chrome marker, extra fine Sharpies (over the chrome for headlights, eyes), and cheap FolkArt acryllics. These were sealed with Pledge Floor Care floor polish about 24 hours after application.
Since you mentioned it was your first time customizing, here is a breakdown of how I disassembled Hound:
I used a Philip's head screwdriver, an X-acto knife (for the head), Loc-Tite super glue, sandpaper for areas where paint would rub during transformation, needlenose pliers, and the Automatic Center Punch. The unsung hero of this project, it's a spring-loaded pin punch used to remove the little metal rods holding some of Hound's parts together. It can be purchased at most Home Depot stores. I would put Hound on a towel, bunched up so no one part was unsupported, and line up the Center Punch with the pin hole. After pushing a few times, the other end would emerge. If it was smooth, I'd punch it back in until the opposite, rougher side of the pin became visible. This threaded end of the pin gives it most of its friction. After that, I could remove the pin with needlenose pliers and set it aside to be punched back in after painting. Other people have used soldering irons to remove pins with only one end visible, but I'm not brave enough for that yet; indeed, I couldn't even remove the pin for Hound's neck.
For Shuffler, I removed the screw holding his face on, cut off his excess forehead with an X-acto knife, and used the center punch to replicate Ironfist's bullet hole.
