Dopo il punto non usare la minuscola.

In Italian, the negative informal command for tu uses the infinitive:

• Usa = Use!
• Non usare = Don’t use!

So non usare is exactly what you expect for don’t use when speaking to one person informally.

Also, Italian often uses this kind of wording in instructions, rules, notices, and manuals, where the subject is left unstated.

Here punto means the punctuation mark period / full stop, not a geometric point.

So:

• il punto = the period / full stop

In other contexts, punto can mean point, dot, stitch, or even point/issue in an argument, but in this sentence the punctuation meaning is the one that fits.

Italian normally uses the definite article in expressions like this.

• dopo il punto = after the period
• dopo la virgola = after the comma

English often drops the article in these cases, but Italian usually keeps it. So dopo il punto is the natural form.

Yes, minuscola is originally an adjective, but here it is being used as a noun.

It is basically short for something like:

• la lettera minuscola = the lowercase letter
• or more generally the lowercase form / lowercase writing

Italian does this quite often: an adjective can stand in for a noun when the meaning is clear.

Compare:

• la maiuscola = uppercase / capital letter
• la minuscola = lowercase / lowercase letter

The singular is natural here because Italian is treating la minuscola as a general concept: lowercase as a writing style or type of letter.

It is similar to saying:

• Use uppercase
• Don’t use lowercase

You are not thinking about several separate letters one by one; you are talking about the category in general.

It can feel like both, depending on context.

Because non usare is the negative tu command, it can mean:

• Don’t use lowercase

But in signs, rules, or teaching materials, this form can also sound like a general instruction rather than a very personal command.

So it may be understood as:

• Don’t use lowercase after a period
• After a period, lowercase should not be used

Yes, that would also be correct.

• Dopo il punto non usare la minuscola
• Non usare la minuscola dopo il punto

Both mean the same thing.

The version with dopo il punto first puts the condition up front: after the period.... This is common in rules and instructions because it tells you the context before giving the command.

This sentence is understandable and works well as an instruction, but there are other natural ways to say it too, for example:

• Dopo il punto si usa la maiuscola. = After a period, you use a capital letter.
• Dopo il punto non si usa la minuscola. = After a period, lowercase is not used.

These versions sound a bit more like general rules.
The original sentence sounds a bit more direct, like a classroom or style-guide instruction.

Yes, in standard writing, Italian normally uses a capital letter at the start of a new sentence, just like English.

So after il punto, you normally write:

• la maiuscola, not la minuscola

Of course, in texting, poetry, advertising, or very informal writing, people may break this rule, but standard Italian follows it.