This should work. To start with, set your spotmeter to the advertised film speed (ISO rating), tape your step wedge to a north-facing window, and measure step 11 (I assume you have a TP 4x5 step wedge). You can get as close as you like and your spotmeter allows you to. This will give you your exposure.
This and the rest is in the book, starting on page 217 with 'Elaborate and Precise' by the way. Step-by-step instructions are on page 221.
Once your ready and still have a question, let me know.