How do I add an 'unsubscribe from emails' link to marketing emails?

[TC83]

Hi, Please can anyone advise how I add an 'unsubscribe from emails' link to my marketing emails? I'm concerned it's a legal requirement as well as courtesy to have this but I cannot find an answer. 
Thank you 🙂

[TC83]

Hi, Thank you, I'm currently just using shopify inbox and gmail.
Tricia 🙂

[TC83]

Thank you 🙂