One of the first things to do now that you are in the midgame is to get Witchery rituals set up.
This can technically also be done in the earlygame, but you will need to do them to progress through the midgame.

For this you will need to craft a piece of Golden Chalk en some Ritual Chalk.
To find out how to set up these rituals, and what items are required, it is highly recommended that you make the Witchcraft: Circle Magic book.

Now that you have the basic materials to do rituals, one of the first things you should make is a Charged Attuned Stone. This will allow you to get your first Blood Orb.

From here on out things in Witchery aren't really gated, so try to figure things out by yourself. Some tips:
 -Spectral Dust can be gotten from killing a spectral familiar, which you can summon with one of the rituals.
 -Infusing yourself kills you, but maybe there is a solution within Witchery...
 -The dream world is impossible to progress in normally, so make a Tinkers' Construct belt to take some items with you.