If you’re negating a particular element of the sentence (an adjective, an object, an adverb, etc), then you should place nicht directly before it.
For example: Sie trinkt nicht gern Bier (She doesn't like drinking BEER)
If you’re negating the entire idea of the sentence, or the verb itself, then nicht should go as far toward the end as possible.
For example: Sie singt nicht (She does not sing)
Phrases like "this is/that is" and even "those are"/these are" are commonly translated to German using "das" (for the latter two in combination with a plural verb). For "this" and "these" you can use "dies" instead, but most of the time you don't.
Note that this is different from the situation, where "that", "this", "those" and "these" are standing directly in front of a noun. Then you have to differentiate and use the fully declined versions.