I don't know about the best time but I did some this week when pruning some stems after last weekends wind.
I mix cutting compost with sand and put in a long pot like the ones used for Clematis or Roses.
Cut stems the thickness of a pencil (they say) but I've used different thickness sizes, about 9 to 12 inches long.
Place around the edge of the pot pushing quite deep into the compost then put in a place that doesn't get direct sun for a few months till you see some new growth.
I have had quite a few roses this way and shared them with friends.
There are probably more correct or technical ways to take them but this works for me.

It looks like we're going to have some fairly mild temperatures & most roses are pretty tough. I'd go with Thorneyside's advice, if they're in pots & the weather does change for the worse you can move them to somewhere hospitable. Even indoors on a non-sunny windowsill

Ive had pretty good success 50% of stems with roses I usually do them in summer when they are putting on growth find it easier to do them in pots as you tend to look after them more than in the ground