Start as you mean to go on.
In any situation, such as a new job, a marriage or as as a parent, it is a good idea to know how you intend to behave and not just muddle through.  This was advice to be pro-active rather than reactive long before those terms were coined.

Mind your Ps and Qs.
This is advice about life rather than writing.  Lower case p and q are very similar, as are B and D and b and d.  Mind your B’s and D’s sounds too similar to make a good memorable phrase, although in English these letters are used more frequently!

Don’t blot your copybook.
In the days of ink wells and pens with nibs that had to be dipped in ink, schoolchildren had a copybook, where they practised handwriting.  If too much ink was put on the nib a blot would result, making a mess of the page.  The instruction is about behaviour and reminds me of a story my mother told me recently.
One of her friends was the daughter of a clergyman and was terrified about giving him her school report as she had misbehaved and been given a “conduct mark”.  Her father read the report and said, “I see you have a conduct mark – that’s very good!”

Use some nous.
Nous rhymes with house and mouse and means common sense.

Play fair.
Fairness is equality.

Show it who’s boss.
This is advice to someone learning a new practical skill.  For example if you are pinning two pieces of fabric together, you have to keep them lined up correctly and be in control.

Do what you can.
Here it sounds as if a task is of Herculean proportions.  Or perhaps it is one of those maths exams where there are too many questions for anyone to complete in the allotted time!

Front panel from a sarcophagus with the Labours of Heracles: from left to right, the Nemean Lion, the Lernaean Hydra, the Erymanthian Boar, the Ceryneian Hind, the Stymphalian birds, the Girdle of Hippolyta, the Augean stables, the Cretan Bull and the Mares of Diomedes. Luni marble, Roman artwork from the middle 3rd century CE.  (Wikipedia)

Photo Credit Wikipedia

Give it your best shot
is another way of giving the same advice, perhaps from the sports field.

Give it your all.
Nothing half-hearted will do here.

Pull your weight.
In a tug-of-war this would be literal advice.  Anywhere else it means the same as the one above.

Don’t be a spoil sport.
If you don’t want to join in and play, there is no need to make everyone else miserable too.

Don’t be a copycat.
There is a song about Carbon the Copycat I used to get rather upset that I didn’t seem to be able to agree with anyone else without being accused of being a copycat.  I was too shy to express my opinion first.

Go into a room as if you owned it.
This is advice about poise.

Stick to your last.
Last here is a form around which a cobbler would make a shoe.  The phrase means to keep to things you really understand.

Keep up your standards.
Sloppy workmanship or bad behaviour is not allowed.

Make your mark.
People who are unable to write can make a mark to indicate that they agree to the contents of a document.  In life someone who makes their mark  usually changes things for the better or excels in their chosen field.

Set a good example.
The responsibility of an elder child!

Give as good as you get.
In a fight or an argument or in response to unkind treatment, this is common advice.  Sometimes bad behaviour should not be tolerated by the person on the receiving end.

Two wrongs don’t make a right.
This is a statement, not a command, but it should be considered when obeying the previous order.

St Paul wrote to the Church at Corinth in 1 Corinthians Chapter 10 verse 13 (NIV) No temptation has seized you except what is common to man.  And God is faithful; he will not let you be tempted beyond what you can bear.  But when you are tempted, he will also provide a way out so that you can stand up under it.