If you want it you merely take it… If taking it is a possibility…
First off, stealing ships and selling them isn’t as profitable as you might think. Stolen ships have to be registered before you can do anything with them. This includes selling them or customizing them. Registering a ship costs some credits.
Furthermore, you can’t steal a B-class or C-class ship if your piloting skills aren’t high enough because you won’t be able to pilot them.
With that out of the way. There are two methods of stealing ships.
The first is regarding ships that are planetside you simply walk up to or fight your way into, clearing the ship, and making your way to the cockpit. From here I’d suggest looting the cargo gold and captains locker first since we’re here to steal anyway.
With that out of the way, merely interact with the cockpit and pilot the ship. You can travel to the closest port and interact with a shipyard technician where you will have to register the ship before being able to sell it.
What about the ship I’ll leave behind if I pilot this one? Don’t worry it will be available to choose from any shipyard technician.
The other method is space combat. Targeting ship engines to kill the ship during the fight. From there you merely fly closer so that you can board the vessel.
Once boarded, fight your way to the cockpit and loot the cargo hold and captain’s locker. Interact with the cockpit undock your own ship and pilot this one.
That’s all there is to it.