Clicking on 'Refit ship' either in a system space station shipyard menu or at your private shipyard will bring up a menu box
You will also note your ship will become transparent.
The left hand pull down panel will allow you to select modules to place on your ship.
The right hand pull down panel will list the modules on your ship to select to remove.
Module selected to be placed must be in storage at the particular location.
Modules selected to be removed will be stored at the particular location.
If your ship is not complete with all required modules, you will not be allowed to undock/fly away so care must be taken.
If you purchase a module from a player, you will need to go to its location, enter refit and fit it to your ship.
Same applies if you have built a module at your own shipyard, you can enter refit and have it fitted.
Certain modules MUST have the same class as your ship class. These modules are Electronics, Warp Drive, Propulsion, Gravity Generator, Computer and Hyperdrive. Armor of any class can be used on a ship provided the ship has cargo space.
The Reactor may be a smaller class but will greatly reduce the capacitor recharge speed. One class smaller is half the size and in general is the smallest you can go and still be usable though capacitor depletes quite quickly. A smaller reactor will give more cargo space. One class bigger is twice the size and makes capacitor appear to never deplete.
Modules required to undock and only need to be class 1 are Salvage Beam, Shield, Ram Scoop, Armour Panels and Mining Beam.
Modules that are not needed at all to undock are the Hyperdrive, Broadside laser, Turret Laser and axial Laser
When placing modules, usually it does not matter where you put them on the ship with just two exceptions. The turret and broadside Laser weapons are placed where you want them to be pointing.