First, there is no such thing as a HDTV antenna. It is a VHF antenna, just like we have been using for 60 years. If you search under DIY antenna, you will see many designs. It is very hard to beat the good old bow-tie. A commercial built antenna will have the details in impedance worked out that a DIY may not. I have built a lot of DIY antennas, Yagi, LP, Rombic, loops. They look very simple, but the little details matter. A lot of good info in from the ARRL antenna compendium. Note they do almost always give 50 Ohm designs, not the 75 you need.
How easy? OK strip about a foot of the shield off a piece of coax. Now you have a quarter wave whip. Everything else gets more complicated.