Why don't you just buy one? Oh..the thrill of the chase..OK
Start with a transformer.
220VAC primary, 12 VAC, 6A secondary
Add a Bridge Rectifier
Add a Filter Capacitor
That gets you an unregulated supply.
For a voltage regulator you can use a zener diode and a pass transistor
or a linear regulator with an external pass transistor
or a switching regulator.
Here is a link to a schematic to a 5 volt power supply that you can use as a guide.
For the current output of 6A you are going to need to use a fairly large heatsink on the linear voltage regulator to keep it from overheating at your maximum load.
Lately I've been using surplus Fan cooled CPU heatsinks for Power supplies with a bolt on bi-metal to switch the fan on above 50 celcius. Just using a 3 terminal regulator and a suitable "boost" transistor, and the entire regulator electronics and CPU heatsink become a compact wee module...(not forgetting appropriate venting). The Transformer and Smoothing caps are now the largest bits in the setup.