Set aside at least 20 minutes every day for practice.
Make sure you are not disturbed by noises other than the ones you create yourself.
Practice sitting down. It's more comfortable than standing and makes it easier to develop correct positioning.
Warm your hands if they are cold.
Rest the guitar on your right leg. If you are left-handed, rest it on your left leg.
You may wish to cross your legs. This will elevate the position of your instrument.
Be careful not to let the neck of the guitar point downward.
Try not to lean over your guitar.
Flex your fingers to loosen your joints.
Check the tuning of your guitar and re-tune it if necessary.
Practice your left hand first, fingering chords and melodic lines.
Warm up by playing tunes that you are already familiar with.
Next, practice new material separately and stick with it for a while. Don't return to your old, familiar songs just because you are getting frustrated and impatient with the new ones.
Finish your practice session by playing music you already know, and polish your skills on it.