Take your favorite modal scale, for example the Pentatonic scale (see Ex.1 below) and find the root position for the note "F". Let me share a trick that I learned as a kid by using mnemonics to remember the order of the circle of Fifths - With sharp keys [#] use - Starting with the key of "F" = Fish, "C" = Can, "G" = Go, "D" = Down, "A" = And, "E" = Eat, "B" = Bait. Fish can go do and it bait = F, C, G, D, A, E, and B accordingly. If you are viewing this in another language you can add a mnemonic that works for your language.
Pentatonic scale examples (root 6th string and root 5th string). [Ex.1]
My intent is to give you an alternative to the often used chromatic (progressing in semitones) progression method of practicing scales. That too is a wonderful tool but my attempt is too broaden your scope and help you learn the circle of fifths which is a music fundamental. Using this you will hear the difference in key intervals in fifths and the difference is very apparent. It will also help you identify a fifth. Being able to identify intervals is also a fundamental and if you have ever had ear training, you will realize identification could be difficult but the ability identify intervals is essential as identifying keys. Nevertheless, lets not get too involved in music theory and keep practice fun.
I find using my favorite scale, the Locrian [7th note of the major scale for those interested] mode, then jumping around on the guitar neck using the circle of fifths as a map can be a wonderful way to keep my playing fresh and fun. The Locrian is only one example; please 7 modes to be thorough.
From the soap box: Learning this is essential if you want to know you $%$##$%. I've played all types of music and as a guitarist you must learn something other than the key of "E". Other musicians make jokes about guitarists because most cannot read music and only play in the key of "E". Try to break that trend and learn because if you don't you can get shot down. I have a real life experience where I was in the college big band, the Professor was a fantastic musician who could play the piano, flugal horn and lead the band. Each year we had try outs, I watched quite a few players who could hold people in awe at a party or at Guitar Center but as soon as the teacher put a chart in front of them - SILENCE. The teacher sent them packing with the immortal words that I will never forget as long as I have my faculties, "Stay home and practice or stay home." *Quote - Joe Davis - !
Joke - How do you make a guitar player shut up? Put a chart in front of him!
Practice! Don't be the butt of a joke!!!!