Given you are not a professional upholsterer, great job, IMO.
There's alot of videos out there to show you how to tighten up the covers. Even leaving them in the hot sun helps shrink them down.
Here's a link below - they use a steamer and heat gun. Be VERY careful with a heat gun as it can damage the vinyl without a doubt.
Just watch how the wrinkles disappear. Not saying yours will be perfect, but I bet some extra work in the nicer weather will greatly improve your results even further.
https://www.youtube.com/watch?v=Rzm4NeSAPyQ
