Hi
This is a good site for all types of building tips
http://www.pavingexpert.com/mortars.htm#conc
Much cheaper if you buy from a good builders merchant than the diy chains
most sell the sand/gravel pre-mixed in 25kg or one tonne bags for easy handling
around here and on most sites i have worked on its called 'balast'
The basic mix would be four/five 25kg bags of balast to 1 bag of cement just go for
the cheapest cement they have.
A 25kg bag of balast should be about £1.15p - 1.25p and a 25kg bag of cement £3.50-£4-50
If you are mixing by hand, mix small quantities at a time to make sure you mix it well,
if you can borrow/hire a mixer its 2-2.5 bags of balast to 1/2 bag of cement.
Mix it together dry before you add your water (a little at a time) untill it looks a consistent colour and just slides of the shovel.
DONT make it too wet - it leaves the mix weak and puts a lot of strain on any shuttering you have and tend to leach out every where.
Fill it in to your shuttering in about 3-4 inch layers and tamp down with a 2" square timber
about 4foot long end on, as you go to give good strength and knocks out any air pockets, this also gives a nice smooth finish, when you strike the shuttering. (leave the shutering in place for 48hours before you strike it) and cover over the top with the empty blastic bags if it looks like it will rain.
If you want to use the shuttering more than once, and/or want a really smoth finish, paint the wood with a thin layer of oil first (i use shuttering oil cause i can get it free, but cheap engine oil is just as good)
And just remember this stuff is heavy to move around and mix especially by hand, so watch your back, its also a chemical which can burn badly, so wear gloves at all times.
Don't breath in the cement dust and keep it out of your eyes at all costs (don't ask how i know that, boy does it sting)
GOOD LUCK hope you get dry weather.