I myself had a similar approach. A few years ago I read through all the Robert E. Howard's Conan stories (and few others of his work) and wrote down every description of landscape that I could find. The final document was about 40 – 60 pages of text excerpts from the 21 stories and drafts & synopses (some times I copied whole paragraphs so the real amount of information isn't that much). I based the map on a combination of the only three map sketches that Howard left and filled it with the details from the stories. (H1, H2 and H3)Just to clarify, the project would have nothing to do with pastiche material. (To the best of my knowledge, this would not only exclude the various speculations about the southern coast and far east, but even the misguided term "Hyboria" itself.)
Although I stayed away from pastiche material I noticed that sticking strictly to REH's word would leave the map very unsatisfying. As you see Howard's sketches include only rough outline of his world, one river and a couple of cities. It poses some real challenges to come up with the rest. I decided to give myself some freedom. So a lot of interpretation was required when piecing the map together and I only excluded stuff from the map if I felt that I couldn't reasonably justify their placing based on Howard's texts.
Some of the geographical features are pretty straightforward to add but many uncertainties arise that require decisions. Border mountains, hills, forests and grasslands were quite easy to place on the map. Rivers were hard, where exactly do they flow when text gives no description? Cities were annoying too. Some of their placement I estimated by travel times based on the clues in the stories, whether horse riding, camel caravan or flying winged beasts. Sometimes I noticed that Howard's sketches weren't “up to date” and some stories gave additional information that required some changes. I had to leave many cities out of the map. For example, we know many Turanian and Shemite cities by name but nothing of their location. Many kingdoms are left completely without descriptions, like next to nothing is told about the landscape of Corinthia or Brythunia. Heck, sometimes we aren't even told where the story takes place. Generally the most well known places are unsurprisingly those were the stories took place.
I mostly sticked to the boundaries of REH's sketches but I expanded the map a bit in the south and south-east to include the stories that took place on the unmapped region. However, things get even more uncertain there. And naturally, almost nothing is known about the rest of the globe and it's continents.
Although there are some mysteries that I haven't been able to solve yet, overall it was quite fun to come up with the map and it was cool to spot some implausible placements from some of the maps done over the years.
I hope you decide to work on your map. I would be very interested to see what you come up with.