In Shop menu buy 1 commander for 180 merit or 10 for 1800 merit.
Commanders have skills for use in Captain, Gunnery, Steering, Communications, Engineer positions. Example Arashi has 500m radar for use in comms position on many ships.

In Barracks menu drag and drop your commanders onto each ship position, select commanders based on skills/ abilities.

Commander skills start working at tier 4+5 at 20%; tier 6 at 40% no stacking; tier 7+8 at 40% with stacking (when 2 commanders with same skill are added together).
Up to including tier 3 use any commanders you want to upgrade as skills have no effect.

In Barracks menu right click a commander, select Items and use naval books to upgrade power from 1 to 10. Naval books are earned in game or purchased in shop 1.5 million silver.
Use commanders in games promotes them to level 10.
It takes many 100's of games to level up commanders.

In Barracks right click commander and select promote to promote them from 1 to 5 star, this gives them more abilities at excellent, legendary levels.
Qualification is used to upgrade commanders. Qualification is obtained by earning merit each day and buying 1 or 10 commander draw in Shop.

Many commanders are not used and there are a few favourite ones that have good reputation with players. Selecting commanders becomes a game you can play to choose your favourites.

In Gallery or Barracks you can select Commanders by Country, ship type. Select by ship type will give you suggestions for commanders whose skills work for that ship, though you can also use other ones.

Examples for Commanders:
Captain position
battleship- Lindeman, Lutjens
destroyer/ cruiser- Mass

Gunnery
(check the main gun stats for damage, penetration and reload of each battleship/ cruiser)
battleship- Macmillan for German or USA ship that may need gun damage and have enough penetration
Lee at 5star or Sandei at 4star if you want reload for Japanese or some UK battleship.
cruiser heavy- US cruiser may need damage commander and Japanese need more reload.

Communication/ Radar
Arashi, Cunningham, Burke, Kazuo, Hoffenstein (battleship).

Engineer/ Steering
Luckner for disguise on any ship.

Ship Commanders
Aircraft carrier- Shelby, Yamamoto, Fletcher, Mitsuo, Doolittle, Nimitz, Spruance, Nagumo,

Submarine- Makoto, Prien, Schuberth, Turne, Doenitz, Kretschmer, Morton, Shinsuke, Luckner (comms- Cunningham, Arashi)

Destroyer, Cruiser light- Mass, Luckner, Muller, Lee, Shotara 4star, Chiaki, Chung-Hoon, MacIntyre, von Musel, Shinsuke, Beattie 5star (comms-Arashi, Cunningham, Kazuo, Burke)

Battleship- Lindeman, Lutjens, Muller, Lee, Sandrei 4star, MacMillan 4star, Darlan, Chiaki 5star, Halsey, Kuznetsov, Yang 5star, Nimitz, von Payne (comms-Arashi, Cunningham, Hoffenstein)

Cruiser heavy- Lindeman, Muller, Luckner, Lee, Chiaki, Nobutake, Yang 5star, Somerville (comms- Arashi, Cunningham, Kazuo)

Other used commanders- Rogge, von Muller, von Tirpitz, Beatty, Kurita, Grey (reduced gun accuracy)