Before you attempt to down-tune your guitar, there are a few basic things you need to take into consideration. Changing strings before changing your tuning, particularly if the strings have been on the guitar for a long period time, will help you stay in tune. Strings get used to having a certain amount of tension; when it is suddenly changed, they will regularly fall out of tune.
If you plan on tuning your guitar more than one whole step down, use 11 gauge strings or higher. Thinner gauge strings, such as common 9 or 10 gauge strings, while perfect for standard E, can sometimes become too loose when tuned down low due to a lack of tension. Often, this lack of tension causes them to vibrate and rattle in an unpleasant way. Thicker gauge strings will supply the appropriate amount of tension when tuned down more than a full step.
Some guitars may require that you replace or widen the nut to hold thick gauge strings. This is a fairly simple process that almost any guitar technician can do.
After you change the strings on your guitar, tune to your desired tuning. Using a chromatic tuner will help you find the correct tuning as well as help you stay in tune as your new strings stretch out. After bringing your guitar to the appropriate tuning, stretch your strings by lightly pulling them up from the fret board and above the pickups or sound hole. Tune the string back to the correct pitch; repeat the process two or three times. Doing this will save you time and frustration later on, especially if you are in a low tuning.
Tuning your guitar down one full step will allow you to not only play songs in a lower key, but tuning to D allows you to simply put a capo on the second fret and play as if you are in standard tuning. A capo on the first fret allows you to play one-half-step down. For this reason, many guitarists choose to keep their instruments tuned down a full step on a regular basis.
Tuning your low-E string down to D while leaving the rest of your strings in standard E tuning makes it possible for a player to form a chord by fretting the first three bass strings only.