In this section, I will divide my card choices up by mana cost; from least to greatest. I will also state why they are so important and why I choose to run the cards I am running.
-Memnite: A classic zero drop to help swarm the field. Good synergy with Springleaf Drum
-Ornithopter: Does the same job as Memnite, but better. Is typically a bigger threat because it's a flyer.
-Mox Opal: This card is pretty self explanitory. It very well may be the most important card in affinity. It allows me to drop out my hand faster than normal and also works very well with Springleaf Drum
-Signal Pest: A good buff to give my little dudes and can help deal a lot of damage the next turn. A good choice for a turn one drop, but that is often determined on what is in your hand.
-Springleaf Drum: A mana source that uses the creatures you dumped out to play more creatures on turn one. With a Mox Opal the right hand can result in a turn one two drop.
-Galvanic Blast: This is a card that doesn't play that big of a role, but is usually useful in an early hand, considering it is basically a guaranteed 4 damage Lightning Bolt turn one. Typically, you see this card swapped for certain cards while sideboarding or remodeling the deck, but a 2-3 if is pretty useful to have; even if you intend on swapping while sideboarding.
-Vault Skirge: It's basically mana because you will typically always pay the life, but it's technically two mana. Vault Skirge is great because it gains me back the life I lost and works as a good flying threat.
-Steel Overseer: This card is good at buffing my small guys. A very valuable turn one drop if possible, and definitely put him out turn one if you can. Strategic tapping is something I have yet to learn with him to get the most value out of casting him, but he is an extremely good card in the deck.
-Arcbound Ravager: Unfortunately, in the end all my artifacts become a sacrifice. Arcbound Ravager is pretty much the win con. He works especially well with Inkmoth Nexus and the sacrifice triggers can be used to get the most value out of your creatures before they are devoured.
-Hangarback Walker: Definitely an important dude in this deck. The instant speed ability activation can be helpful when blocking and he synergizes especially well Cranial Plating and Arcbound Ravager.
-Cranial Plating: An essential card to every affinity deck. It plays the role of (obviously) a win-con, where you can equip it to Inkmoth Nexus for lethal damage quite early in the game, or on your typical Ornithopter or Vault Skirge.
-Etched Champion: This creature is especially good at being evasive and is a very good beater with Cranial Plating and can easily be an alternate wincon.
-Master of Etherium: This dude is good for buffing smaller 1 and 0 drops and those smaller dudes buff up this bigger dude. Symbiosis my dudes.