This is a great idea I saw a few years ago. It worked well then and works great now, with the new plastic gutters that are available.
Way back then gutters where wood or steel. The guys I saw used wood and oiled it so the concrete did not stick. They still had some problems removing the curbs but it worked!
What you do is get a couple of 10′ lengths of plastic gutter.
Make some braces to stop it from stretching open when you fill it. These can be made with 1″ x 6″ boards placed along each side and braced with stakes driven into the ground.
You will also need to cut a couple of pieces to cover each end. These can be screwed on to the brace boards.
Oil the inside of the gutter and fill with concrete!
In a couple of days remove your 1 x 6 boards and you will be able to take out your new curb. They come out easy because you can stretch the plastic a bit to help with removal.
Don’t try to drag the curb around for at least a week, cover it with plastic and keep it wet for 7 days. It should be cured enough to use buy then if your temperature is in the 60 – 70 degree range. If it is colder you should wait longer, or until late Spring or Summer!!
It is also a good plan to add a length of 1/2″ rebar to the concrete when the mold is about 1/2 poured. This will make it far stronger and less likely to break when you are manoeuvring it around.
These are VERY heavy so try to make them as close as you can to where the will be living. I always try to do them so all I have to do is roll them over and they are in place.
A good aggregate type of concrete mix is best for this. Or simply use redi-mix from your favourite building supply. Be sure to follow the directions for mixing it. If you make the mix to liquid it will have no strength and probably break before you get it into place.