Whenever you need to drop a ball to get it back into play, you first need to find your Reference Point. This is often your Nearest Point of Complete Relief, but it could be where you played from previously, you might be going back on-line, or using another option.
Mark your Reference PoInt with a tee peg, and then measure out the extent of the Relief Area using the longest club in your bag which isn’t a putter (so probably your driver). You then drop from knee height directly into the Relief Area and the ball must stay within that Relief Area. It is now back in play (left hand picture).
If the ball lands outside the relief area (right hand picture), or hits you or your equipment before it lands, or it is not dropped from knee height then it has not been dropped in the right way and you will have to drop it again.
If it rolls out of the relief area (middle picture), it will need to be re-dropped.
There is a bit more on Dropping the ball here: