The easiest is if you get to Cancun* first.
From Cancun it is an easy transfer to Cozumel.
See image below 👇

From Europe: there are direct flights to
Cancun from Madrid, Paris,
London, Amsterdam, Frankfurt,
Istanbul, London and from
Malmo and Copenhagen with TUI,

From New Zealand: fly to Cozumel via Houston,
or to Cancun via Los Angeles or San Francisco.

Rest of the world: find a flight to any of
these destinations
and then to Cancun or Cozumel.

From Mexico: you probably know how to get to Cozumel.

Full list of airlines that operate to Cancun here:

https://www.cancunairport.com/airlines-directory.html

Make sure that the Cancun airport is the close to Cancun,

there are two!

*Unless you fly to Houston,
there are direct flights to Cozumel from there.