Close to the fret. In general (this is not always the case), make sure that your finger is as close to the fret as possible without actually touching the fret wire. This will ensure that the string does not buzz or sound muffled when played.

One finger per string. Be sure that only one finger touches each string. Often you will find one finger slightly touching a neighbouring string. This is particularly common with the more difficult chords (like the G major chord). This is one other cause of muffled or buzzing strings. Take some time after constructing the chord to make sure that each finger is only touching the necessary strings and not resting on neighbouring strings.

Relax. Relax your hand so that it is comfortable when constructing the chord. Re-position your wrist for each chord so that you are comfortable and so that your fingers can sustain their position (without cramping) while you strum.

Look at each chord diagram closely. Spend some time positioning your fingers to look like the images. You need to memorize the name of the chord as well as the position of the fingers so choose a few at a time then gradually add another. Later, try to become quicker at changing from one chord to another.