Cut gun away from Zemo's hand.
Cut that arm off of Zemo.
Glue it back on with it now going straight down his side.
Cut the other arm off now.
Glue it back on with it facing forward, as if holding something in front of him.
(you may need to shave/cut it in a way that fits flush at the connection point)
Not required, but I cut a couple of slivers off the connection point and glued them onto his hip area to form more of the skirt look he has. This could also be accomplished by just building up paint or ignoring it entirely.
Then you can shave off some of his side(the side with the arm out forward/his right) to give it bit more of a curved shape. This accentuates his thinner waist leading to the wider chest above and the skirt like fabric on his wider hips. I also shaved off a bit of the other arm(the one hanging straight down/his left) to make it look thinner leading to the wider area of the shoulder above and the wrist below, both of which puff out a bit more.
Then you can shave the top of the white part at the top of the boots that fluff out, to make them look more like tights that go down into shoes.
This is also optional, but I did it in an attempt to better capture the likeness of Pennywise, which is to cut the legs off from the "skirt". This is because the legs by default are spread out, whereas Pennywise's legs are usually closer together. So it's fairly minor, thus optional. If you do it, then you'll glue them back on the base, closer together, then glue his torso back on top.
Now you'll want to cut Zemo's head off, and then paint his body all white/off white.
Cut/pull Slapstick's head off.
Cut some of the extra hair off to shorten it a bit. Just refer to a picture of Pennywise for reference to get the best desired results.
Glue the head onto where Zemo's head was.
Paint the body a light/mid tone grey, leaving the ruffles around the neck and wrists and his gloves white.
Add 3 red blobs/circles for the red clown balls in the torso. I used a bit of built up paint blobs to give them a bit of depth instead of just flat circles, but whatever works fine.
Paint his shoes a darker grey, with 1 red blob on the top of each foot near the toes.
Paint his face and forehead white. He has a pretty noticeably big forehead as part of his look, so I built a bit of excess paint even covering up a bit of the hair on his forehead as well as receding a bit, so refer to the pictures to ensure it looks correct as you don't want his hairline so far down his head.
Paint his hair an orangish color.
Referring to a picture of him or the figure, paint the red paint parts of his face including the end of his nose, his mouth, and the lines that extend from the edges of his mouth up across his eyes.
Paint his eye sockets black.
Add a yellowish/orangish dot for each eye.
Then I just dirtied it up a bit with a wash(
very watered down black paint applied over his outfit then you dab it down leaving some of the sunken in areas darker and dirtier looking) and put some subtle horizontal lines going down his tights.
Then it's complete. Feel free to add a balloon of some sort if you can find something to use, but for simplicity here I left it out.