Tune your guitar. A guitar in standard tuning from low (the sixth string) to high (the first string) is tuned E-A-D-G-B-E. To tune to open G, drop the sixth and first strings down to D and the fifth string to G. This can be accomplished by sounding the D string and retuning your E strings to the octave. Next, sound the G string and tune what would normally be your a string to that octave. Alternatively, you can use an electronic tuner.
Choose which finger will wear the slide. The pinky is often the choice for slide players because it allows three other fingers to fret the strings. Some players, such as Duane Allman and Derek Trucks, found their third finger to be more comfortable. Bonnie Raitt opts to wear her slide on her middle finger. Spend a little bit of time wearing your slide on different fingers before deciding which works best for you.
To sound notes, place the slide directly above the fret. When you fret a guitar with your fingers, you must press down behind the fret to sound the correct note. To play the correct pitch with a slide, you must place the slide exactly above the fret. This is because you are sliding over the strings rather than pressing them down, and the slide acts as a fret.
Get familiar with playing in the open G. The tuning is set up to play a G chord without fingering any strings. Playing in that key will allow you to focus on the basics of using a slide and spend less effort in learning where the correct notes are. Although it is important to learn to play in every key, starting in G will help you advance faster.
Play chords. Playing chords in open G tuning is easy. Strumming all of the strings open will sound a G major chord. Great sounding major chords can be made by playing the second, third, fourth and fifth strings together at the same fret. Played open and at the 12th fret will be G chords, at the fifth fret C, and the seventh D. Practice playing a chord progression with those chords.
Play some licks. Effective licks can be played by hitting the fifth string open and sliding from the second fret to the third fret on the third and fourth strings. Also, try hitting the fifth string open and sliding from the 11th to the 12th fret on the first, second and third strings. When switching to the C chord, cool licks can be found by sliding up from the third to fifth fret on the second, third and fourth strings. Sliding down from the seventh to fifth fret on the first string and the sixth to fifth fret on the second string work great as well. To play off of the D chord, play those licks two frets higher.
Use vibrato. Vibrato is part of the signature sound of slide guitar. To produce the effect, slide up to your desired note and shake the slide rapidly from side to side as the note sustains. Vary your shaking speed as well as the area you cover and listen to the differences your changes make. Practice this technique often and you will eventually have a controlled vibrato that will be very useful in your playing.