Sign in to ads.google.com.
In the top bar go to "Tools & Settings → Measurement → Conversions".
Click the "+" button to add a new conversion event.
Click "Website" for the kind of conversion.
Fill out the setup of the event as desired (this will be different depending on what you want to track).
On the next page, click "Install the tag yourself"
In your form, add the "Global site tag" under Configure → Analytics → Page load scripts.
To trigger the conversion you'll want to use the "Page load" event snippet. It depends on what you want to track as the conversion as to where you will put this snippet.
Track conversions on page load (Pro and Agency): Place the snippet straight after the global site tag in "Page load scripts".
Track on submission (Pro and Agency): Place the snippet under "Successful submission scripts".
Track on custom success pages (Agency): Place the snippet in custom HTML block.