export PATH=$PATH:/mypath1:/mypath2 in ~/.profile. Means "add :/mypath1:/mypath2 to what is already in $PATH." If you need the entire system to be aware, set or update it in /etc/environment with PATH=...
I do feel like setting environment variable on linux is not as intuitive as on windows, but after I setup my workflow, I realized I never have the need to manually set any environmental variable besides in flatseal.