You probably have a mask or two in your stash. Or you can start with a shape. Then use the eraser tool with an opacity of less than 100% to soften the edges. You can also add other elements to it. However, you need to flatten them together before you start erasing - you also need to fill it with black. What I did was group the items, copy them off to the side, and then flatten the original, erase, etc. That way if you need to go back, you can delete the flattened "mess" and use the grouped items to start over. I suggest you build the mask over your picture so you can see if it will cover it.
When you have the mask built, select, Ctrl-C, then go the photo, add a mask effect, but in the drop down box for the mask selection, do a Paste and it will pop in your mask. If you don't like it, backup a few steps and repeat.
This one is not very blendy, but I created this mask in A5. The small butterflies are part of the mask and were a separate element that I added to the stack of things in the mask. If you have any questions, ask away.