Why a direct review link?
Happy customers are willing to write a review, but drop off when they first have to find your business on Google Maps. A direct link opens the window with the stars straight away. Google itself recommends sending customers a link or QR code, and you can also find one in your Business Profile under "Get more reviews".
How does the link work?
Every place on Google Maps has a unique Place ID. The link search.google.com/local/writereview?placeid= followed by that ID opens the review window for exactly that business. Google also returns such a "write a review" link for every place in the Places API. If the link does not work, use the one from your own Business Profile.
Where do you use the QR code?
At the till or reception, on invoices and delivery notes, on a card with each delivery and in your thank-you email after an appointment. Ask every customer the same way: Google prohibits selectively asking happy customers or offering anything in return for a review.