How do I cover this wrought iron metal chair?

http://jonpeters.com/refinish-make-new-seats-for-wrought-iron-chairs/

Cut plywood to fit seat, upholster it with foam, fabric and staple gun. Maybe weave the back with sisal or twine or something?

This girl had the same open frame you do, and here is what she did, but you could even paint the wood or stain it if you didn't just want it plain:

http://thriftychicken1.blogspot.com/2013/10/something-old-is-new-again.html

There was a similar issue asked here a couple of years ago. Style of chair was different but the spaces that need filled are the same:

https://www.hometalk.com/diy/repair/furniture/q-seeking-advice-on-how-to-replace-seats-backs-on-iron-patio-chairs-9967290

I didn't find the suggestions to be all that helpful but maybe you will find some inspiration there.

If this were mine I would seek nearby upholsterers for their solutions. This chair is nice enough to invest in and a plywood seat and some sort of weave for the back would leave it functional but crude. I'd invest in the upholsterer and have a uniquely beautiful piece that doesn't have to come in every time it rains.

Here's a good idea-https://www.youtube.com/watch?v=2HNOdvNiReo

Maybe something like this

The man on You Tube had a complete and expert workshop, I doubt J.R. Kern has one. S/he can fasten a board to the seat by having it cut slightly larger and using a metal item that has two flat places (with holes for screws) on either side of a half round that would go over the bar. However the man in the video showed no such fastener, in fact I think his seats just rested on the frame!. Rush or webbing could also be woven around the frame. Frankly though I do not think covering this would be a job for an amateur.

i just got another notification for your message even though I already replied so I'm confused. I apologize if this is redundant

How bout these