First things first. you need to get some blood. In order to harvest blood you need some Bone Needles. these are crafted like so:
Craft yourself some Glass Bottles and hit a mob with a needle. This will consume the needle and the bottle, but you will get a Jar of Blood in return.
Next up, the Necronomicon.
With the necronomicon you can create yourself a Summoning Altar! Place 2 cobblestone and a plank in a row (in the world, not the crafting bench) and right click the plank. You still can’t summon a minion yet though. You need to get some souls first. How? With a Scythe, a Blood Scythe.
Use this weapon to kill some mobs and make sure you have some glass bottles left in your inventory. If you use the Scythe to deliver the final blow, a bottle will be filled with a soul.
Back to the Summoning Altar. You should have some bones, rotten flesh, gunpowder and string. You should see a couple of slots in the Altar. The left one is for blood, the right one is for souls. Place your mob drops in the center to create a minion. If you are happy with the way your minion looks and you have supplied the altar with blood and a soul, you shift right-click the wooden part and the minion will spawn. if you aren’t creative, the altar will use up your blood, soul and mob drops.