Watch the The G - C - D7 Progression online guitar lesson by Richard Gilewitz from Jump Start Fingerstyle Guitar
It’s fairly common to have a 7th chord be the last chord of a progression, as it is here. Let’s use this strumming pattern:
1 & 2 & 3 & 4
Down – Down – Up – Up - Down
Note how I use my index finger as an anchor for playing both C and D7, so keep it in close to the guitar neck.