MAS don’t sell the cheapest tickets over the phone. So you have to book online for them.
While you are making the booking and before you pay you have a chance to reserve a seat or simply move straight on to payment. What they don’t tell you is that they charge you extra to reserve a seat. So you click on the reserve a seat button, choose one, and then you find out the charge.
You can’t check in until 48 hours before. But MAS send you an email about 48 hours before with a link, you click on it and put in your booking reference to check in. At this stage you are allocated a seat, but you can change it without any fee. You can print out the boarding pass, and / or have it sent to your phone. With the latter you get a web link, which on clicking gives you the pass with QR code on your phone. The check in was simple, fast and easy.
At the airport, before you enter security someone checks your passport / ID and boarding pass. Simply showing the mobile phone screen with the boarding pass showing was fine.
Then, at the gate just before boarding, they want to see your ID or passport and the boarding pass. This time they take your phone from you briefly to scan the QR code and then you proceed.
It all works fine – except if your mobile chooses that moment to run out of battery, restart due to system update, etc. Having been in the IT industry, I believe in backups. A printout as a backup is a good idea.