Watch the Strumming Options online guitar lesson by Rich Maloof from Acoustic Guitar For Beginners
Your strummed chords may sound muddled or like they have wrong notes in them if you don't know where the bottom note is. Watch the chord grids so that you know whether you should strum 6, 5, or 4 strings. If the grid shows an X next to a string, that means it shouldn't be strummed.
Once you get familiar with your chords, you'll get used to strumming them from the 6th string down, from the 5th string down, or from the 4th string down.
An entirely new type of right-hand movement combines plucked and strummed strings. For this you will definitely need to know which is the lowest string you can pluck, so mind those chord grids and watch out for strings that should not be played at all (marked with an X).