Liners are easier to handle when the weather is warm and it isn't blowing a gale, so I'd say now was a pretty good time, and then you can have it finished in time for the frog spawn in the spring.
Don't put fish in or they'll eat the wildlife - some idiot put a fish in my allotment pond, I never did find out who it was, though I have a pretty good idea. It brought duck weed with it too, so I was very unhappy about that.
give it wide margins and grow lots of marginals so the wildlife can come and go with ease - dragon flies need marginals to lay their eggs on - such a privilege to see the imagos climbing back out after a couple of years lurking.
