Part 1 -- sorry, I couldn't explain it all in 90 seconds so I had to do three parts. Before masking choose your image to have the sky replaced -- if you bracket your exposures on most overcast days the exposure with the sky just burnt out will be the most effective. Alternatively use the dodge tool to make the cloudy sky white or use an exposure adjustment brush in Lightroom or Camera Raw to overexpose the sky. Although you can mask the sky in pretty well this way over an image with sky detail, it is more effective if the sky layer is on multiply or darken over a base image with a white overexposed sky -- then you will be able to keep fine detail like TV antennas or fine foliage. Copy your replacement sky to a layer above, try to use a sky with nice gradation from horizon up. A lot of days I can shoot a clear patch on another angle to be used but I also have a large library of skies to choose from. Skies with some puffy cumulus or wispy cirrus clouds seem to work well with houses shot on overcast days. The steps then are: - On the image layer choose the blue channel (command/alt + 3) - Select all and copy - Activate all channels (command/alt + ~) - Go into quick mask mode (type Q) - Paste copied channel - Exit quick mask mode (type Q again) -- your copied channel will become the active selection - Select the sky layer then click on the mask button on the bottom of the layer palette (the active selection will become the mask) - Option/alt click on the mask thumbnail in the layer palette and you will be able to work in the mask channel - Command/alt + L to get levels and clip the highlights and shadows to get a high contrast silhouetted image -- you just want to lose the sky detail and darken the main part of foliage, some tonal detail will help foliage blend into the sky - Type O to get the burn tool (shift + O will cycle through the tools until the burn tool is selected, or alternatively I prefer to just keep it on the burn tool and use the hold down the option/alt key to temporarily get the dodge tool whenever I need it) - Use the burn tool to go over the edges and increase the contrast wherever needed. - Quickly lasoo select the remaining image and fill with black to complete the mask - Finally click on the little chain link between the thumbnails for the sky image and sky mask so that you freely move the sky image into position - Change the blending mode of the sky layer to darken or multiply so that the fine detail in foliage remains, sometimes you might want slightly blur the mask to soften the transition. - Alternatively, if your base image still has sky detail, keep your blending mode on normal for the sky layer and just select the edge of the skyline from base image and copy it to a new layer and layer that over the top in multiply mode and drop the opacity to suit. It can get very fiddly on dark cloudy days and you are trying to keep things like power lines in the image so you don't misrepresent the property. It may look complex but with keyboard shortcuts or all this recorded as an action you can drop in a replacement sky in well under a minute quite easily. It is still very annoying though if you have a lot of images to replace the skies in though. It is summer down here in Australia and I don't care how hot it is as long as the sky is clear -- I hate having to replace skies!!!

Part 3 -- sorry, I couldn't do it all in 90 seconds so I had to do three parts. Before masking choose your image to have the sky replaced -- if you bracket your exposures on most overcast days the exposure with the sky just burnt out will be the most effective. Alternatively use the dodge tool to make the cloudy sky white or use an exposure adjustment brush in Lightroom or Camera Raw to overexpose the sky. Although you can mask the sky in pretty well this way over an image with sky detail, it is more effective if the sky layer is on multiply or darken over a base image with a white overexposed sky -- then you will be able to keep fine detail like TV antennas or fine foliage. Copy your replacement sky to a layer above, try to use a sky with nice gradation from horizon up. A lot of days I can shoot a clear patch on another angle to be used but I also have a large library of skies to choose from. Skies with some puffy cumulus or wispy cirrus clouds seem to work well with houses shot on overcast days. The steps then are: - On the image layer choose the blue channel (command/alt + 3) - Select all and copy - Activate all channels (command/alt + ~) - Go into quick mask mode (type Q) - Paste copied channel - Exit quick mask mode (type Q again) -- your copied channel will become the active selection - Select the sky layer then click on the mask button on the bottom of the layer palette (the active selection will become the mask) - Option/alt click on the mask thumbnail in the layer palette and you will be able to work in the mask channel - Command/alt + L to get levels and clip the highlights and shadows to get a high contrast silhouetted image -- you just want to lose the sky detail and darken the main part of foliage, some tonal detail will help foliage blend into the sky - Type O to get the burn tool (shift + O will cycle through the tools until the burn tool is selected, or alternatively I prefer to just keep it on the burn tool and use the hold down the option/alt key to temporarily get the dodge tool whenever I need it) - Use the burn tool to go over the edges and increase the contrast wherever needed. - Quickly lasoo select the remaining image and fill with black to complete the mask - Finally click on the little chain link between the thumbnails for the sky image and sky mask so that you freely move the sky image into position - Change the blending mode of the sky layer to darken or multiply so that the fine detail in foliage remains, sometimes you might want slightly blur the mask to soften the transition. - Alternatively, if your base image still has sky detail, keep your blending mode on normal for the sky layer and just select the edge of the skyline from base image and copy it to a new layer and layer that over the top in multiply mode and drop the opacity to suit. It can get very fiddly on dark cloudy days and you are trying to keep things like power lines in the image so you don't misrepresent the property. It may look complex but with keyboard shortcuts or all this recorded as an action you can drop in a replacement sky in well under a minute quite easily. It is still very annoying though if you have a lot of images to replace the skies in though. It is summer down here in Australia and I don't care how hot it is as long as the sky is clear -- I hate having to replace skies!!!