Hi davewalter,
This depends on whether you want to redirect a stock screen to your custom screen (in the case of modifying a stock screen), or navigate to a custom screen from any other screen.
To redirect, go to Edit App for your app, find the stock screen you want to replace with your custom screen and click Redirect (you'll have to first click the > arrow on the tile). You can then select your custom screen, and it will be called in place of the stock screen.
To navigate to your custom screen from any screen, you can add a button with a Go To Screen action that points to your custom screen.
Hope this helps.
- Justin