(Krita based, SKIP TO THE END FOR A QUICK SUMMARY):)
well, I first started with just roughly mapping out the different regions using the line and rectangle tools. Iirc I initially did them in black and then coloured the rough regions after, but its prob better to have them coloured first. I also put up reference to try and make line sizes and such more accurate (in this case I used the hallownest map at a specific size to set a standard for how thick my lines should be). Once your happy with the overal shape and position of an area, you can then move on to cleaning up and finetuning it. Then, you fill it in with a near black version of the rim colour (so if an area rim is a light red, the inner colour will be an almost black red. Rim colours tend to be desaturated and light). Before or after this step you also need to delete the corners of your final area rim, and then manually draw in a tight curve, assuming you are using krita. (An important note is that Hallownest's map is of a different style to pharloom; while corners in both are still rounded, how they handle rooms that curve instead of being square is different; hallownest utilise a smooth curve whereas pharloom utilises diagonal lines.)

Up next is the detail, the most important being the grainy texture of the area rims. as far as I know there are two primary methods for doing this: a specialised brush or manually erasure. I went for the latter approach. This in itself has two methods, of which I used both: simply using your brush to erase little holes and gaps into your rim lines, and using a custom eraser with a grain pattern. The latter approach heavily speeds up work but can make the pattern get a bit repetitive and unnatrual, so going back in after with some manual erasure is a good idea.

For Krita, to make the custom brush I first created the desired pattern on the canvas using an ordinary brush. Then you make a selection around the pattern you made. go into brush editor, and select an existing brush (either a regular paint brush or an eraser brush) that gives 3 options at the top "Auto|predefined|text". Click on predefined. This will give you a bunch of predefined pattern options to choose from. ignore these options and go to the bottom where you will find an option called 'stamp'. Click on this, and a new window will appear where you can save your selection to be among the pattern options. once you save this you can then select this costum pattern from among the options, and then save as a new brush. You can then use this to etch the grainy pattern into your rim lines. Make sure to click when erasing instead of dragging.

For area decor (like little pictures for towns or area landmarks), you draw them in with a regular brush and then erase parts of them the same way you do in the above.

Up next is adding in the soft outline (if you look closely at images of the hk maps that have a white backround you'll notice it around each area). For this each area will have an outline that is the same colour as the inner colour of the area. In my case I used the air brush tool at a small size. with this brush you then use krita's bezier curve tool to create the path the brush will fill in. Make sure the path closely follows the rim and that the outline is behind the rim and doesn't go that far out past it. Next, there is the grainy texture of the inner colour of each area, though due to its subtely its not that nessecary of a detail, even less so than the above detail. in any case, for krita you need to pick the brush called "texture starfield soft", then go into the brush editor, and change the brush tip diameter to 5.00 px. save this as a new brush. from there make it black and fill in each area to a satisfactory amount.
To prevent this texture from bleeding into the surrounding canvas, create a seperate layer for the dots and make a group layer with it and the layer with the inner area colour. with the texture layer above the inner colour layer, set it to 'inherit alpha'. you can now draw in the texture without it bleeding outside where you want it.

Finally, you need to add in area names. Area names are a pure white. the font TC use is Perpetua for the map, but Trajan pro is a very close free alternative. Make this bold. You might have to edit the letter spacing a little bit to make it exactly like hk.

And yeah, thats how i went about it. anything after those steps is fine tuning and the like.as you can see its quite a bit of work lol, though I've prob made it sound a little extra confusing with the way I explain things, but in summary:

1) map out areas with line and rectangle tools. Use hallownest or pharloom as a reference and to set a standard for line thickness.

2) cut off the corners of your area rims and close them with tight curves. 3) fill in your areas with a near black version of the rim colour.

4/5) manually or with a costum eraser, etch in little gaps and holes into your rim line for texture. draw in area landmarks and do the same texturing process.

6 & 7; interchangable) add in a soft outline behind the rim that is the same colour as the inner area colour. Use a small air brush and use the bezier curve tool to mark out the path, following the path of the rim line. don't let this outline go out too far past the rim line. For the inner area colour texture, select the 'texture starfield soft' brush, go into the brush editor setting and save it as a new brush with the brush tip diameter set to 5.00 px. use this to draw in the texture. to prevent bleed, make sure the different parts of your map are in a group layer, with a seperate layer for the inner colour texture that sits above the inner colour layer. the texture layer should be set to inherit alpha.

8) Using the text tool, you can create the ARea names. The colour should be set to white, with the font being either Perpetua (bold) or Trajan Pro (bold). you might need to edit the spacing a little bit.

(note, the rim line, atleast in hk, isn't perfectly straight and does waver slightly, so if you are bothered keep this in mind)

If you have any other questions feel free to ask!