This is some good advice.
As far as I am aware, the original is explained best here in the Tate Etc. magazine archive where the piece is identified as being Peter Fischli and David Weiss's How to Work Better from 1991. Since then - in addition to inspiring from studio walls - it has wandered the internet, often in the visual form you see at left.
One of the things I like best about this particular form, and which gives it more weight with me than the text alone, is the imperfection of it. It is a good reminder that it's better to get something out into the world than to endlessly tweak on it seeking perfection.
The words are great too.
Do one thing at a time.
There's a key Expediter principle; you will achieve more working on multiple projects if you give them your full attention for set chunks of time than if you flit between them rapidly. The chunks don't need to be large - even 15 to 30 minute sprints can be hugely productive. Just focus and don't give in to the "I'll just take a quick peek to see if there's new email" urges.
Know the problem.
There are many ways to interpret this, but one which I find valuable is to confirm with myself what it is I am trying to solve or achieve. What is the outcome I am seeking? I've heard GTD coaches phrase this as "What would done look like?"
Learn to listen.
Simple, right? Nope. This one is a lot harder than it seems and critical to success in all aspects of life. Really shut up - mouth and mind - and really listen. Then think. Then respond.
Learn to ask questions.
Assumptions can bite you in the butt later. Ask, clarify, confirm. Even when you're working on something for yourself, ten minutes spent unpacking and spelling out your expectations into a brief journal entry can both vastly improve the finished work and steer you clear of avoidable problems.
Distinguish sense from nonsense.
This is the outcome of listening and asking questions. What actually works in this situation? What doesn't fit?
Accept change as inevitable.
This is true of our projects, our companies, our culture, and most definitely ourselves. Much of the nonsense we deal with results from trying to shoehorn the no-longer-current into a changed situation.
They are inevitable and they are valuable. Denying them is a far worse mistake than anything else that could go wrong and interferes with learning.
Say it simple.
Omit needless words.
Brevity is the soul of wit.
"Do only what is necessary to convey what is essential. [C]arefully eliminate elements that distract from the essential whole, elements that obstruct and obscure... Clutter, bulk, and erudition confuse perception and stifle comprehension, whereas simplicity allows clear and direct attention."
- Richard Powell, Wabi Sabi Simple, quoted in Presentation Zen
"The first rule is to keep an untroubled spirit. The second is to look things in the face and know them for what they are."
- Marcus Aurelius
Calmness is essential to clear perception and appropriate reaction.
Yes; it helps. Being happier tends to make all the other parts easier. So keeping your spirits untroubled is a good investment.
And even when you aren't feeling your best, be nice. That's a prudent investment too.