Approximation of ancient landmasses using current ocean floor data
over that short a time there would be very little change in the land itself, all you really get is a sea level change due to changes in the ice sheets, continental movement is barely noticeable.
such measurements and maps already exist.
here is the earth 12,000 years ago.

Here is the same map with the changes highlighted.

sources for other maps
https://www.fieldmuseum.org/pleistocene-sea-level-maps
http://antiquity.ac.uk/projgall/sakellariou334/
Or you can even use this interactive map, which already does what you want to do, and compare it to past sea level.
As for why projecting backwards is easy, you may want to look up the term &quot;continental shelf&quot;.