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Step 1

Here's a pattern that I have decided might look good without the typical 90 degree corners on the inside, I'm going to give it a slight smooth rounded corner:

Step 2

I start out by creating a new Raster Layer and create a rectangular selection pattern. Normally I would create this new piece in the same color as the primary pattern itself, but for illustration purposes I'll use another color:

Step 3

Now I change to Selection type: Circle, create a corner to corner selection and hit delete on my keyboard:

If needed, I use the Move Tool (M) and place it into a better looking position. If I would have created it in the color of the original pattern it would look like this:

Step 4

Then, I would repeat the same step for each corner that I wanted a smooth rounded corner or slight curve on. When I have completed the modifications that suit me, then I merge all of my pattern layers together and apply my effects: