I'm part way through my extension and when i did mine, i marked out the correct dimensions (carefully), dug down to the required depth (depends on your subsoil i suppose), ordered the concrete mix, poured it in, levelled it (carefully).
Once it was set the builder put the first courses of brickwork in, then we insulated the floor along with a dpc membrane, then ordered the flooring concrete, poured it in, levelled it (carefully), let it set, then built the rest of the brickwork up.
For depths of footings/concrete etc, wait for a pro to come along and help. The above is just the order we did things in.