The paired conjunctions both . . . and, either. . . or, neither. . . nor, and not only . . . but also require parallel structures.

I know both where you went and what you did.
Either Mark or Sue has the book.
The tickets are neither ir my pocket nor in my purse.
He is not only an excellent student but also an outstanding athlete.

The following is not parallel and must be corrected:

He wants either to go by train or by plane*.

It is not correct because to go by train is not parallel to by plane. It can be corrected in several ways.

He wants either to go by train or to go by plane.
He wants to go eitherby train or by plane.
He wants to go by either train or plane.

When you are using these paired conjunctions, be sure that the correct parts are used together.
The following are incorrect:

I want both this book or* that one.
Either Sam nor* Sue is taking the course.

These sentences are incorrect because the wrong parts of the paired conjunctions are used together. In the first example, and should be used with both. In the second example, or should be used with either.