Here are some instructions from the fireslide package:
More than anything, being in tune is the difference between sounding like a learner or a pro.
• make sure your guitar is in tune — really in tune
• trust your ears and listen to what they tell you
• hold the fireslide over and parallel to the fret
• as you practise, tune each chord before moving on to the next
• you can use standard guitar tuning — I do, I dislike retuning
• or you can use another tuning, it’s up to you
• if you use the fireslide in alternate tuning, I’d love to hear from you!
listen to your ears!
• “good tone” means a clear, robust, and sustained sound
• more mass/inertia in the slide makes better tone (use a new lighter and listen to the difference)
• the greatest inertia is in the middle of the fireslide
• tone is best in the middle of the fireslide and poorest at the ends, so play with the middle of the fireslide over the strings you’re picking
• if the slide is touching the string it’s doing its job
• use light pressure so the slide is just resting against the string(s) you are picking — relax!
• avoid pushing down too far, which results in ugly collisions with the neck — the dreaded “clack”
• experiment with where you pick the strings
• practise slowly and listen to your sound
• vibrato should sound natural and personal and should reflect the mood of the music
• fast vibrato is energising, slow vibrato is dreamy and in between is in between
• find the note(s) first, then add vibrato — it’s more interesting and more likely to be in tune
• feel the inertia of the slide interacting with your hand as you move it back and forth along the strings
• learn to vary the vibrato rate and width
• if you’re fretting, you can let your fretting fingers slide along or keep them in place and move the slide independently — whatever you prefer
• vibrato shouldn’t be used to cover up poor tuning
• you can let the strings sing undamped or you can break things up by damping the strings
• finger-fretted notes can be damped by lifting the fretting finger a bit or touching with the right hand
• slide-slid notes can be damped by touching with available left-hand fingers or the right hand