Credit to PS3Trophies for the guide.
Go to the moon and open your popit with SQUARE.
Get an object from the goodies bag – the icon is a bag with a solid white surface – which are on the second page.
Place the object in the environment, then go back to the tools bag – the other bag icon with the gears in the middle – and on the second page down at the bottom you will find Weaponizers.
Simply select whichever one you want and attach it to the object.
Go back and get your popit cursor – the icon that’s a stringy line with an arrow point – and select the weaponizer with SQUARE.
Edit it however you please, then save the object by grabbing it with the capture object tool found in the goodies bag.
After that, place a custom weapon launcher found on the third page of the tools bag.
Select it with the popit cursor to edit what’s inside and put your custom weapon to fire out. This will net you both Custom Object and Arms Creator.