Here is an example of a long sentence:

A policy is needed which defends non-commercial cultural goods, which incorporates the development and education of all kinds of artistic personnel, which introduces children and young people to the best of our cultural values, and which promotes the highest cultural values and achievements.^{Footnote 1}

43 words.

There is, of course, no single correct way to rewrite this sentence, but here is one version, ignoring other problems with the sentence and focusing only on length.

A policy is needed which defends non-commercial cultural goods and incorporates the development and education of all kinds of artistic personnel. It needs to introduce children and young people to the best of our cultural values. It also needs to promote the highest cultural values and achievements.

21, 15 and 11 words.

Remember, a sentence should express one idea only, using a subject and verb. This idea may need more words to complete it, but it is safe to say that if you are thinking of two ideas, you should have at least two sentences. You should be able to read it aloud in one breath, or possibly with just a second quick breath where there is a comma or semicolon.

Get rid of the extra words
The second way to make your sentences shorter is to cut the number of words used. This is one of the most useful ways to make documents clearer. Most of us use more words than are necessary.

A man who uses a great many words to express his meaning is like a marksman who, instead of aiming a single stone at an object, takes up a handful and throws in hopes he may hit.
—Samuel Johnson

I am sorry this is such a long letter, but I did not have the time to write a short one.
—Mark Twain

This report, by its very length, defends itself against the risk of being read.
—Winston Churchill

I try to leave out the parts that people skip.
—Elmore Leonard