How do I grow hostas in a pot?

This looks like sunburn to me. I've included photos of mine below. Mine are 3 year old plants that were potted in 12" pots straight from the nursery and they were about the size of yours at that time.

We recently moved to a new place and I've been moving mine around trying to find a spot where they won't burn. It took a while, but I found it. In the meantime they did burn and you can see some of the remnants of this on mine. They look exactly like yours.

At my previous place, the plants were against a fence where they only received a few hours of very early morning sun and then shade for the rest of the day. They loved it!

Now, they get dappled sun from trees with almost full sun around noon for 30-45 mins. They are doing okay with it and it's the best I can do for them without building a special shade contraption for them. Keep in mind, hostas can grow in full shade if necessary.

Additionally, on the photo of the plant with red outlines you can see signs of heat stress. In the summer, we get 95-100'+ temps during the day. So, even though they had plenty of water, transpiration rates were high and they dimpled from loss of water. This will also happen if they get too dry when they are growing and new leaves are emerging. Bloom stalks will also twist on some varieties if they are too dry when they start to emerge.

A couple of suggestions - remove the blossom stalks when they are done blooming so the plant can direct it's energy toward the leaves or growing more roots. Otherwise, it's trying to nurture seed pod growth. Also, I see roots on one of your photos. I would add a thin ( an inch or two ) layer of compost or enriched dirt to the top to give the roots some protection from the sun.

So, move them to a shadier spot. Go for early morning or evening sun. Or move them to the other side of your tree. I need to keep mine out of full sun from 9am to 5pm because I'm in the high desert at 6800 ft. So, the sun is super intense here. You may get away with more exposure depending on your location.

Hope that helps.