how do I hook up older TV+digital cable+dvd player?

Answer 1

you might already have one though
a vcr can be a modulator
plug the cable box/dvd player into the input of the vcr
and plug the rf output of the vcr into the tv
you should have to select 'line 1' or 'av 1' or something
a vcr will also tune cable channels
but you might have trouble tuning digital cable channels

Answer 2

you must run the signal through a RFmodulator which can be purchased at any electronics store.