To create a badge on Roblox, first go to create.roblox.com. Next, click on the game you want to add the badge to. On the sidebar to the left, click Associated Items. Click on Badges and click Create a Badge.
Choose an image for your badge. Make sure your image file is in one of the following formats: .jpg, .png, .tga, .bmp. Then fill out the name and description and click Create Badge. You can create 5 badges for free every day. Additional badges will cost 100 robux each.
That is how you create a badge on Roblox. However, you will need to script the functionality of your badge to make it work inside of your game. Click here to learn how to script a Welcome badge that a player will receive when they join your game for the first time.