1) Check whether planning consent is needed (it probably isn't). Have a plan drawn up and obtain planning if necessary.
2) Use the plan to get 3 builders to quote or:
3) Dig for foundations and fill with concrete
4) Construct walls in brick or block. Single skin is enough if it is merely attached to the outside of the house, not accessed through it. Use DPC at 100m above ground level
5) Construct sloping roof structure using 4x2. Cover with marine ply, then finish using flat roof covering materials.
6) Dig out inside to suitable depth and fill with 100m rubble, then blind with sand, put DPM down and cover with 70mm screed.
7) Put in window and door frames.