STEP BY STEP CHANNEL CUT OUTS
Make complex selections using channels in Affinity Photo to replace a sky fast
01 COPY AND PASTE
Open channels_before 01 and 02 into Affinity Photo. Go to the 01 image, copy (Cmd/Ctrl+C), then go to the sky image and paste (Cmd/Ctrl+V). Press Cmd/Ctrl+J to make a duplicate of the trees image, so that there are two tree layers and one sky layer below.