Learn English – What does “in steps of + a certain number” mean

In this context "certain" means a predefined subset. Usually it's used when the subset is fairly arbitrary, and to shut down discussions on how that subset is defined.

If the infinitive form was used after verbs that began with the letter "d", then the book would simply say:

The infinitive form is used after verbs that begin with the letter "d".

But because the set of verbs where it is used is fairly arbitrary, and generally only learnt through experience, the book probably doesn't want to delve into the rules, preferring to list a few common examples, and say:

The infinitive form is used after certain verbs.

I might say "my party officially starts at 9pm, but I've invited certain friends round for dinner at 7" if I didn't want to get into the details of which people I invited to the pre-party dinner party.

Not exactly. The meaning is closer to "live your everyday life". Let's look at it another way. Here's an entry for go about from TFD:

go about
2. To execute some routine: From my office on the top floor, I could observe all the city's workers going about their business.

Your day just refers to your typical day, your daily activities. So another way to interpret "go about your day" is execute your daily routine.