Summarize:

Its icon resembles a white ghost on a yellow background. If you aren't already logged into Snapchat, tap Log In and enter your username (or email address) and password. Doing so will open your profile. It's in the top right corner of the screen. You'll find this in the "Additional Services" section. It will turn green. Now you'll be able to use filters in your Snaps! If the switch is green, filters are already enabled.
Open the Snapchat app. Swipe down on the camera screen. Tap ⚙️. Scroll down and tap Manage Preferences. Slide the Filters switch right.