(note, there are likely many ways to do this, this is just what worked for me but it should work for you; but nonetheless: proceed at your own risk!
- DO you already have qgis installed ? -REMOVE IT.
- Are you already use Ubuntugis-unstable ppa? If yes, go to 3. if no, go to 2A.
I know i'm removing a lot of packages right now but package conflicts are the primary reason that packages don't work in the first place. 2A] uninstall gdal, liblwgeom* (this is to prevent package conflicts with ubuntugis-unstable PPA).
3] I've been using the ubuntugis-unstable PPA. Add that repository if you haven't already. This has newly released items while not having to build new packages every night yourself. These packages might introduce bugs for you.
add the ubuntugis-unstable PPA more assistance with install PPAs at https://help.ubuntu.com/community/Repositories/Ubuntu#Adding_Personal_Package_Archives_.28PPAs.29
do sudo apt-get update
4] Reading over https://qgis.org/en/site/forusers/alldownloads.html#debian-ubuntu
4A] input the keyring code that immediately follow "In case of keyserver errors..."
4B: you'll need to add the correct qgis repo which would be the following line:
deb https://qgis.org/ubuntugis xenial main