Hi there, Norman,
If you have a branch of B&Q or Wickes near you, you should be able to find a 'How To' leaflet on concreting. Otherwise, try Google.
The type of aggregate available will vary a bit according to your location. For instance, up the Thames valley, it's usually pit ballast from gravel-pits on the flood-plain. In South Devon, where my parents lived after they retired, it used to be gritty stuff rejected by the Cornish china clay industry while here in East Hampshire, some builders' merchants supply aggregate dredged from the sea bed. It gets washed and graded (aka sieved) to separate different grain sizes. In other parts of the country it might be crushed stone or even ex-demolition crushed concrete. It all depends on the local geology. If you ask your local supplier I'm sure they will explain what is common in your area.
When I lived in Essex, I used to use 'as dug' ballast which is everything from ¾" down (except that it was washed to remove the clay) so that was coarse plus fine combined. Some recipes will call for the coarse and fine constituents separately - you combine them during the mixing.
You need enough fine aggregate (sand and fine gravel) to almost fill the spaces between the coarse aggregate, otherwise you'll need to add more cement. I agree with Jeff's advice about the aggregate to cement ratio but it isn't too critical, you don't need a laboratory balance to weigh the ingredients! (Well, actually, I expect you'll find most recipes measure by volume, not weight.) Actually, a builder's shovel is a recognised measure in some older tables of weights and measures. I'd suggest that for your quantities, a domestic coal shovel will do fine.
If you're mixing by hand, you turn it over three times dry and then three times wet, adding the water little by little. If you're using a mixer, you need to add at least some of the water first.
You shouldn't add too much water, else you'll get shrinkage. If you're going to vibrate as Jeff suggested, it should almost seem too dry as you put the mix into the mould (shuttering) and the vibration will mobilise it. (Have you seen that video of liquifaction of the soil beneath a building foundation during an earthquake?!?) If it doesn't mobilise, you can always tip it out and add a little more water, mix and re-pour. However, you mustn't let it dry out for at least three days. The chemical reaction of the concrete setting requires available water until the concrete has cured, so visit it regularly with the watering can or keep it covered with wet sacking and in the shade.
Once it's cured and really dried out, there might be some slight shrinkage but you could maybe introduce some polyester or epoxy resin into the cracks between the concrete and the metal.