How do I add effects to my double jump ?

When I wrote a double jump script I was able to make my character jump two steps but I didn't know how to add animation and effects :( Please help me.

make an animation using roblox's animation editor, I suggest you do some research because we can't help you with requests

What I would do is create a script that changes the jump animation ID of every player to your animation ID you created. Then I would create a function that activates when the player jumps effects will be instanced into the HumanoidRootPart, but then destroyed. I'm not going to tell you the script though.