Add custom translation variable

[Chill_user]

Need just some info about how to add custom translations to Prestashop 1.7.

What I like to do:

-Add custom translation variables to my theme (classic) to change it later from backoffice -> Translations

-use it in .tpl files like a variable as {l s='trasnlation'} or something like this.

Any tutorial or guide? Maybe some topic with explanation?

 

Thanks a lot!