Strumming and Flatpicking...

 

Basically, strumming is an instinctive action that either comes easily or it doesn't.  Most guitarists strum with a pick (plectrum) because it gives a crisper sound and a cleaner attack, but it is not unknown for player to strum with their fingers, thumb or a thumb pick.  As with all right hand technique it is important for the overall feel of your playing that you keep a steady fluid strum while alternating between downstrokes and upstrokes.  Upstrokes produce a sharper sound because the top strings are emphasized and a downstroke produces a fuller sound because the loser string receive most of the impact of the stroke.

At the same time you should be aiming to emphasize accents on various beats.  Until you can do this it will be impossible to establish either downbeats or backbeats.  You can use your right hand palm to mute or deaden certain strings by resting it on top of the saddle.  You can also use your left hand to dampen strings.  Dampening not only provides space between chord and notes but also helps in establishing accented beats.

Obviously practice makes perfect.  Too often new guitarists are so concerned about the left hand, trying to play some tricky riff to impress their friends that the right hand technique is forgotten to the point where they can't even strum a few open chords in perfect time.  If you cannot play a simple song perfectly you should not be spending your time trying to play something more difficult.

Flatpicking is a style used mainly in acoustic folk, bluegrass and country.  Although any playing using a flat pick could be described as "Flatpicking", there is an established style using open string chords and single string bass notes that is called "Flatpicking".  In its simplest form it consists of alternating the bass notes of an open chord.  Usually it is the root and the fifth of the chord.  So take a G major chord.  G is the root and D is the fifth.  You would finger a G chord and alternate the bass string between G and D while strumming the top strings in between.  The technique evolved partially because it gives the left hand fingers time to form the rest of the chord while the bass note is being played, making fast chord changes a little easier.

When you have mastered the alternating bass notes, the next step is to introduce bass lines and fills to connect open chords in a progression.