Then you'll need to buy the second MS350-24X.
This one comes with a 0.5m stacking cable, so if you would still have that one that was delivered from the existing switch you can fully stack them.
Else if you've lost it you'll need to order one MA-CBL-40G-50CM. Basically you need two in total to complete the 2switch stack.
The switches need to be close to one another.
If 0.5 meters is not enough you can order longer stacking cables of 1 or 3 meters.
2x MA-CBL-40G-1M or MA-CBL-40G-3M
The stack config in dashboard is really straightforward but you will need to bring the new switch online without stacking it first and make sure the existing and the new switch have the same firmware.
Then you'll need to power off both devices and connect the stacking cables in the back and startup the switches again.
Then you'll have a potential stack which you'll just need to provision and voila.
You can now start double uplinking every access switch for redundancy and extra bandwith through LACP port channels.