My old TV has antenna/cable in and video, audio L & R inputs. How would I connect this TV to my Directv box, dvd, vcr and modulator RF?

You may not need a modulator. They are great for connecting DVD Payers with no Coax
Out, to TV's with only Coax in, then you connect red, white and yellow in to modulator
And it outputs on channel 3 or 4 selectable.
You need to connect output of Box to input of TV.
Set TV to Channel 3 if using Coax, Select Video Input if using RCA Cables.
Yellow is Video
Red is Right
White is Left.