I don't actually like the look of HDR pictures, especially of landscapes. I always find they look fake. But for learning purposes I decided to create an HDR picture with gimp. I took three pictures of Kirkjufell in Grundarfjörður, with normal exposure, -2 and +2 exposure. I did some basic white balance correction and normalizing for each image.



Then I opened the pictures as layers in gimp (File → Open as Layers). with the normally exposed as the base image on on bottom and the overexposed image on top. I opened the layer dialogue (Windows → Dockable Dialogs → Layers) and chose "Add Layer Mask" for the underexposed picture by right clicking on it in the layer dialogue. I chose a Grayscale copy of layer as maks.
I then did the same for the over exposed image, but inverted the grayscale layer mask (Colors → Invert).
Layer masks determine which part of the layer is transparent and which opaque, with black being full transparency and white full opacity. Thus the lighter parts of the underexposed and the darker parts of the overexposed image are layered over the base image. I then played around with the layer modes using screen (other possibility = addition or normal) mode for the underexposed and burn (other possibility = multiply or normal) for the overexposed. See here for an explanation for layer modes in gimp. I played around with transparency of the two layers until the result was less fake looking. The I merged the three layers together and corrected the colour balance again. Here is the result: