It's better to keep your guitar in the case when not in use. It protects the guitar from minor knocks and spills. Make sure the case is in a safe place where it's not likely to get knocked over.
A humidifier in the case, or in the soundhole is a good idea. Don't get it too wet. Guitars want to be in about 60% humidity. Keep your guitar away from heat sources (furnace vents, radiators, direct sunlight).
Except for very short periods of time, don't keep your guitar in a car trunk. (It can turn into an oven and melt your guitar's glue joints.) Guitars are comfortable where people are comfortable.
It isn't necessary or even desirable to de-tune your strings between playing sessions. You should de-tune the strings if the guitar is going to travel (air line etc.) or be shipped, or if the guitar is going to be unplayed for six months or more.
Wiping your strings with a dry cloth after playing can remove moisture and extend their life. Check your strings by running a finger over them, they should feel smooth with no corrosion or rust.
Clean your guitar with a soft cloth. Occasionally use a guitar cleaner/polish never standard furniture polish. A drop of lemon oil rubbed in every six months on the fingerboard will help it stay moist.
To change strings: It's best to loosen one string at a time, replace it and move on to the next. This keeps the guitar under normal load. On steel string pin bridges, gently remove the bridge pin using either the special notch for that purpose on a string winder or use the jaws of a diagonal cutter pliers (which you'll need to cut off the end anyway) lifting carefully under the knob on the top of the pin. When replacing the string, put the bridge pin in loosely and pull the string up until the brass ball on the string end rests on the underside of the top.
Thread the string through the hole in the tuner (itís easiest if you rotate the tuner so the hole is parallel to the strings). With some slack in the string, bend it toward the inside of the tuning head, bring it under itself (so when the tuner is tightened it locks onto itself) and tighten the tuner to near pitch. (Since the other strings are in pitch, you should be able to tune the new string up fairly quickly. Strings will stretch when new so you'll have to keep re-tuning it until they hold pitch. ( Gently pulling the string (like a bow string) above the 12th fret will speed the process.)
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