Theory:

Should is a verb. It is used to indicate obligation, duty, or correctness, typically when criticizing someone's actions.
After using should,  you use the base form of the infinitive.

Should + Verb (base form of infinitive)
Example:
You should go now.
How is the verb "should" used in sentences?
1. To give advice, a recommendation or a suggestion.

This is to say that it is the right thing to do or the correct thing.
Example:
You should make an appointment with the dentist.

2. Expresses that a situation is likely in the present.
Example:
He should have the letter by now. I sent it three weeks ago.

3. Expresses that a situation is likely in the future (prediction).
Example:
I posted the cheque yesterday so it should arrive this week.

4. Expresses an obligation that is not as strong as must.

Sometimes should is used instead of must to make rules, orders or instructions sound more polite. This may appear more frequently on formal notices or on information sheets.
Example:
You should be at work before 9.

5. It was expected in the past but didn't happen (should $$+$$ have $$+$$ past participle).

This expresses the idea that the subject did not fulfill their obligation in the past or did not act responsibly.
Example:
You should have given your boss the report yesterday when he asked for it.

6. Not fulfilling an obligation (should $$+$$ be $$+$$ verb -ing).

This expresses the idea that the subject is not fulfilling their obligation or is not acting sensibly.
Example:
They should be working instead of playing around.

7. Sometimes should is replaced by ought to without a change in meaning. Note that ought to sounds more formal and is used less frequently.
Example:
They ought to stop doing that. (= They should stop doing that)
Shouldn't

We use "shouldn't" to advise not to do something, usually because it is bad or wrong to do.
Example:
You shouldn't throw your litter onto the street.
Reference:
commonersguidetolife.blogspot.com
http://www.grammar.cl/english/should.htm
www.telegraph.co.uk
www.zakat.sg
www.pqsl.org.tt
www.connectedisolation.com
https://commons.wikimedia.org/wiki/File:Sample_cheque.jpeg