Take the old strings off :-
Thoroughly clean the fretboard with lemon oil (espcially if it's rosewood). Let it soak in,if the wood is dry,then wipe off any excess with a cloth.
    Meanwhile,clean the guitar all over with whatever you think best; I tend to use a damp cloth,then I wipe off any excess moisture with a cloth / rag, then apply (sparingly) polish ( if your guitar has a polished / lacquered finish ),and buff to a shiny finish.
Throw away the plastic bridge-pins,and replace with solid brass ,or spun aluminium,ones.The difference in tone and / or volume can be astounding !
Trim off the new strings,leaving some margin for error (!) and,having threaded them through the holes in your tuners,bend the ends back the OPPOSITE way ,whilst taking up the tension. Sounds odd,I know,but----it works !
Tune up to somewhere near what you are looking for,then lay the guitar on your lap,and ,with your fingers,pull up HARD on the string / strings you want to tune.
             Drink some /several beer/beers.
    Re-tune.
          Snip /trim excess from ends of strings at machine-heads with side-cutters. Do NOT repeat NOT curl several inches into a whirly thing.Looks really silly !
Wash blood from pin-pricks in fingertips, have another drink,put guitar in case,and go to bed with a pretty woman (or an ugly one--the end result is usually the same ).
    Or ----go to music shop / instrument dealer,and ask him to fit new strings.