For the cellular connection on the MX itself you’ll need a SIM and a USB modem to get going, you’ll probably get both of these from your carrier - and, yes, it’s pretty straightforward. However, the cellular USB connection doesn’t give you the same feature set as the full WAN ports (e.g. it doesn’t do SD-WAN) - it’s more to just maintain some sort of connectivity to the MX and site.
If by a pair of MX84s you mean an HA pair then just be aware that the cellular connectivity doesn’t kick in until all uplinks (across both MXs) have failed, so it really is a ‘last-resort’ type of thing.
There is a good document here that explains it, https://documentation.meraki.com/MX/Cellular/3G%2F%2F4G_Cellular_Failover_with_USB_Modems, and also lists the USB modems Meraki have tested, but others work too.
If you’re wanting to use cellular as a more robust, second/redundant WAN link to the MX then you might actually want to look at the option of getting a MG21 and connecting it to the WAN2 of the MX. This will give you more of a ‘full-featured’ WAN link and also has the advantage that you can position the antenna away from the MX, towards the cell tower.
