Learn German: Mein Plan ist es, mehr Sport zu machen.

In German, es often acts as a placeholder. Here, it stands in for the whole infinitive clause that comes after the comma (mehr Sport zu machen). It is like literally saying My plan is it, to do more sport.

Where exactly do the words zu machen belong in the sentence?

In an infinitive clause, the word zu goes immediately before the infinitive verb (machen), and together they are pushed to the very end of the clause. That is why mehr Sport comes before them.

Is the comma before mehr Sport zu machen required?

Yes. When an infinitive clause refers back to a placeholder word like es or a noun like Plan in the main sentence, you must use a comma to separate the clauses.

Why do we use machen with sport instead of tun?

While both verbs can translate to the English verb to do, the standard German phrases for doing sports are Sport machen or Sport treiben. Using tun here would sound unnatural to native speakers.