Instead of easing, I would try wrapping it like a package. I think it's called a box corner.
Good luck!

Sounds like you already have the straight sides stapled. That's good.

I start by testing the fabric.....flatten your hand over the corner and slide it down....how does it wrinkle? Can the wrinkles be pulled and smoothed? Now pull downward....harder.....any play? Does it stretch at all? Is it smoothing out?

Now you have to make a decision....a wrinkled corner...a single pleated corner(cutting the pleat to help flatten it).....or multiple pleats.....

I am not sure what a slipseat actually looksl like. In any case, lots of tutorials on YouTube for covering dining room chairs. I hope this helps!

Generally, a slip seat is the bottom of a chair that can easily be removed for recovering. One example of a slip seat:

I don't have the sides totally stapled yet. I have them tacked down with a few staples to see how things will fit. The problem with the corners is the fabric is so thick that I cannot use the techniques nicole describes, which is what I've done in the past when recovering slipseats.

I think I need to make some sort of large notch so that I only end up with 2 layers of fabric at the corner. What scares me about notching the corners is I only have one piece of fabric for each chair. It came in panels rather than yardage (see photo below). I bought it some time ago and may not be able to replace it if I mess up.

It ravels pretty easily. If I notch it, how close to the chair bottom corner should I notch? If I don't do it properly, I'll end up with a hole at the corner.

Should I reinforce the corner of the notch? If so, how? Stay-stitch at the corner? If so, how do I keep the stitching from showing?

Or should I take it to a professional upholsterer and let him deal with it? This really seems too simple not to just do it myself. I'm convinced that others have faced this same issue and found a satisfactory solution. But right now, my mind is boggled at what to do.

My sister tried to help me yesterday and got so frustrated with the thickness, she said I should upholster the seats in plain linen and use the panels for something else! I'm not ready to give up yet.

Check out the tutorial on the blog little green notebook. How to upholster bench corners...I think this will help you.

Thanks, kitch4me. That was helpful. I need to do a practice run, or two, with a similar weight fabric.

Don't know how to describe my thoughts, but could you cut away the corners of the wooden board for the seat to accommodate the fabric instead of cutting your fabric? If you cut the corner off you might be able to fold each side back on itself and create a pleat on the corner with the extra space created. It would be something to try without ruining your fabric, which is very pretty.

Finally got the project started.
{{!gwi}}

Two down, two to go. It is taking me about an hour each. It involves lots of hacking away excess fabric; lots of FrayCheck; and a hammer. As I was trying to get the pleats on the corners to lay flat, something tickled my brain. I don't know where it came from, except I know it wasn't an original thought, but I picked up the hammer and used it to flatten the pleats in the fabric on the corners. It helped immensely.

I tried the technique used by the writer of The Little Green Notebook, but I never could make it work right.

Your technique seems to have worked, looks great!
Love the fabric!