You still have to do this for the other group. Simply repeat steps 1 - 4 for the the other treatment condition. The SSw is a good estimate of within group variability or error. However, it has a problem. The larger the number of subjects, the larger will be SSw.
More subjects mean more deviation scores.
Certis paribus, this means a bigger SSw. Obviously, this is not what you want, so you will need to divide by the degrees of freedom. This will turn the SSw into the average amount of squared variance for each subject. It is abbreviated MSw. Now we need an estimate of our effect.