It's quite easy actually.
The MR76 has two sets of ports. The two on the top are for 2.4 GHz and the two on the bottom are for 5 GHz.
The Meraki antennas themselves are the following (you also have single band antennas but we will focus on the dual band ones since you can use them on any antenna port.
MA-ANT-20: This partnumber contains 2 stick antennas that screw directly on the N-type connectors on the AP and provide the typical 360 degree donut horizontal coverage if you wall or pole mount the AP. This antenna has the least range but provides the full circle.
MA-ANT-25: This partnumber contains 1 patch antenna with 2 short coax leads that connect to or both 2.4 GHz connectors or both 5 GHz connectors. Do not connect one antenna to one 2.4 GHz connector and one 5 GHz connector!! This antenna provides 180 degree coverage with moderate distance. If the intention is to provide both bands you should order 2x this partnumber and connect one antenna to both 2.4 ports and the other to the 5 ports. Aim them in the same direction.
MA-ANT-27: this is roughly the same as the previous as far as cables and connections go. However this antenna has a 120 degree coverage and significantly higher range than the previous one.
In normal use cases where each outdoor AP is wired to the network you usually chose 2x the same antenna partnumber per AP and install them like this. If you only want to serve one band you can omit one of the parts but you will need to buy weatherproof N-type caps to prevent moisture ingress.
In a specialized use case for example the AP will not be wired directly to the network but will be used as a mesh AP then you could have one MA-ANT-27 aimed at the root AP on one band and provide client coverage on the other band using MA-ANT-20 instead.
