1) Create a new composition and add a background image.

2) Check Effects & Preset in the Window menu at the top.

3) Search for CC Rainfall in the Effects & Preset panel and add the effect to the image layer(Background image layer or new solid layer)

4) You can see that the Rainfall effect has been added to the image.

5) You can set the properties of CC Rainfall in the effects added to the layer or in the effect controls panel.

Set the amount, size, speed, direction, color, etc. of rain.