I did a stoop last year for a buddy, 5' x 9' in PA. finished had popped off due to freezing. Putting a sealer on after finish will help to seal out moisture penetrating the surface. This will be found at Cement supply for the trades, not home depot.
I busted it out by hand, poured back with Home depot 5000 (psi rated). Turned out well, a project like this is best done when it warms up. It will take forever to set up in the cold, not worth the grief. Above 50 during the day, start early am, do not let freeze for a few days. This likely not pressing, do wait for it to warm up, throw a sheet of 1/2" CDX ply over and cut nail it to the old concrete that is coming out... if it is a safety hazard. This bridge its use until the weather warms. When you pour, be sure to have a friend or two over so that the cement is mixed in a short period. You dont want it setting up while you are still pouring. I dont know the demensions, If it is a stoop, will not likely justify the minimum for a short (small) load of concrete. I dont like the premix, but at $500 minimum for small amts... i do use premix concrete. I dont like it because it is laborious to mix. With the additives it has, the 5000 is pretty good stuff. Be sure to have a friend who has some idea of how to finish, this is a craft. If he uses the terms dry or smoothers... keep looking. Concrete cures or sets up and is troweled or finished, never smoothed...

Can cover with a tarp, we have used straw and a tarp. have finished under a huge blow up tent with space heater before and in the rain. When the rain paused, we pulled the tarp and replaced it before it started raining again. We poured, had a huge tarp 10mil, we stuck the space heater under one side. This was a military pour with a high spec floor for a PX freezer. finish was good, only complaint was from the saw guy who came out the next morning to cut the relief, crack control cuts after the pour. It set up so hard, he thought it had been poured a few weeks earlier and said he had to use a cure blade. I nearly had to pull out the mud tickets to prove we had just poured the slab.
This may be more than what you asked for, but gives an idea it is a little more involved...