Its a long answer. Id post to home improvement. Some karma chaser will tell you everything.

Astragal in middle Weatherstriping at bottom Weather stripping at outside May need some putty added where glass meets wood Make sure casing is caulked good

That should stop air migration. Single pane glass sucks so if you do those things and still sucks, may need to plastic over the glass. May also want to pop off casings install spray foam then reinstall.