I can provide a short guide on how to do so below: Note: It is currently not possible to save your game during a mission. The devs have said this is due to bugs corrupting saves and damaging the mission you were playing on. Hopefully, as the game gets closer towards completion, this feature will be enabled. Whilst on the strategic overview containing the campaign map amongst everything else, you will find a button called 'menu'. You can either click on this or press the 'escape' key.