An easy process that can give your landscape photos an interesting effect.
Step 1: Shoot a series of photos as you would for a time lapse video sequence. For the smoothest cloud textures, limit the gap between exposures to 1 to 3 seconds. Try to keep the sky from being over-exposed. You may need to take a few additional shots of the land to get the correct exposure balance (mixing the two will be covered later).
Step 2: Starting with the earliest exposures, load a group of the series into your composition as layers by choosing File>Scripts>Load Files into Stack... I used groups of 30 photos to avoid lengthy processing times and then repeated the process with the resulting intermediate images. You may want to experiment with how many layers you add in this step.
Step 3: Select all of the layers.
Step 4: in the main menu, choose Layer>Arrange>Reverse. This will order the layers so that the earliest exposures are on the bottom of the stack, so that the newer exposures aappear "above" the older exposures.
Step 5: With all of the layers still selected, change the layer compositing mode from Normal to Lighten.
Step 6: Save the resulting image as "xxxx-intermediate-01", or whatever name you can keep track of.
Step 7: Repeat steps 2 through 6 until all of your groups of photos have been processed into intermediate compositions.
Step 8: Repeat steps 2 through 6 using the newly created intermediate compositions.
Step 9: Add a new top layer consisting of your best single exposure of the "land" portion of the image (which may have been exposed differently from the sky time lapse images).
Step 10: Using a soft edged eraser tool, erase all of the sky from the top layer, revealing the sky composite beneath.
Step 11: Adjust exposure of the sky and land layers as desired. When satisfied, save the multi-layer composition, then flatten the image and save in your preferred format.