That's not how I'd go about it.
More like
1. Shifter in the 7 position.
2. Loosen cable anchor.
3. Pedal. Observe if derailleur moves chain onto small cog.
4. If not, check for bent derailleur, hanger, bad canle, etc.
5. If so, pull slack out of cable and tighten anchor.
6. Adjust cable tension with barrel adjuster til shifting is good.
7. If largest cogs not useable, check for cable/housing corrosion or damage, derailleur or shifter damage or cable routing problem