Add fonts assets

Make a folder inside /assets in the main app and then make another subfolder in assets named fonts:

/assets
   /fonts

Now put all the downloaded fonts inside the fonts folder as shown below:

/assets
   /fonts
      /Schyler-Regular.ttf
      /Schyler-Italic.ttf

Register the font#

Open your pubspec.yaml file.
Add the fonts info under the flutter section. Indentation is mandatory.

fonts:
    - family: Schyler
      fonts:
        - asset: fonts/Schyler-Regular.ttf
        - asset: fonts/Schyler-Italic.ttf
          style: italic

Set the font using fontFamily inside the Text widget as shown below:

Text(
  'Hello world',
  style: TextStyle(
    fontFamily: 'Schyler',
  ),
)