If you're talking about the photo filter feature on Instagram, it's actually pretty simple. When you go to edit a photo, you'll see a list of options on the left side of the screen. One of those options is Filters.
When you click on that, you'll see a bunch of different filter options that you can apply to your photo. Some of the more popular ones include Normal, Clarendon, Gingham, and Juno.
To apply a filter, all you have to do is click on it and then click the Apply button. That's it!
If you're looking to add a bit more of a creative touch to your photos, you can also check out some of the other editing options that Instagram offers, such as adding text, changing the color of your photo, or even playing with the perspective.