Open Photoshop and load your collage. Click on the "File" menu, click "Open" and find the collage file you saved. Highlight it and click "Open."
Click on the elliptical marquee tool or the rectangular marquee tool -- depending on what shape you want to feather around your image. Typically, you would use the same shape as the collage -- so use the elliptical marquee tool if the collage is round; use the rectangular marquee tool if it's rectangular. Click inside the collage -- so that you don't select the edges -- and drag the mouse until you've covered this same area on all sides of the collage. If you use the elliptical marquee or rectangular marquee tool for your feather, skip to Step 4.
Alternatively, select the lasso tool in the toolbox to draw a shape around the area you want feathered. The lasso tool is ideal for collages of irregular shape; the lasso tool provides a free-form selection. While the lasso tool is activated, drag the lasso within the collage -- the edges, which you don't select, will receive the feathered effect.
Click "Select" from the menu at the top of the page. Click the "Feather..." option. Set the "Feather Radius" to whatever value of pixels you want. Typically, the higher the number entered, the greater the spread of the feathered edge will be on your collage; this will make the overall collage lighter. The lower the number entered here, the more concentrated the feathered edge will be, just on the image's perimeter, and the sharper and vibrant the overall collage will be. Click "OK." You will notice the marquee's shape change slightly.
Copy the area by clicking on "Edit" and selecting "Copy."
Create a new document by clicking "File" and selecting "New."
Paste the selection into the new document by clicking on "Edit" and selecting "Paste." You will now see the feathered edges on your collage.