Moutaz,
I don't know how much details you need but I am listing all major activities.
Tank top:
Plate ends preparation,laying of plates on ground as per layout, Tacking of plates, marking, cutting, welding(both sides), edge bending, reinforcement ribs(if required)
Tank Bottom:
Same activities as for top.
Shells:
Same activities as for top with difference that edge bending will be replaced with shell rolling. No of shells depend upon height of the tank.
Accessories:
Manholes and nozzles will be fabricated as per design.
Fitup of tank technique is different for different sizes. For small tanks, fitup can be started from bottom ie bottom is placed on ground or foundation and then shells are fitted one by one and finally top is fitted. For big tank, it is better to start fitup from top ang keep on lifting the top while adding shells and finally bottom of the tank. This method is used to avoid scaffolding work. Discuss with your fabrication team about it. Nozzles and manhole are fitted after complete fitup of the tank. Don't forget to list down internal welding work as it takes very long time depends upon plate thickness and number of passes.
Syed Hussain