Reviews and citations have started to become an important source of credibility for your web site and business.
Some methods available:
Your own Web site
Create a review page on your web site from where you can send the link to your clients to ask for a review.
On the review page you can place the various review links where a client can click on the link and go straight to the review site.
QR Code
Download a QR code from your Google My Business profile.
A client would scan the qr code then go straight to your Google My Business Reviews page to write a review.
Place the QR code of your web site.
Print the QR code to place at your business.
Print the QR code on a card that you can give to your clients to scan.

Google NFC Review
NFC Review is a device where a user can tap there phone on the device and then go straight to your review page.
It comes in various formats like desktop and business card formats, that are available from Takealot and Amazon.
Do a search on Takealot and Amazon and you should find similar results like in the examples below.

