I know what I actually need to do, I just need to sit down and start working and once I've started it's much easier to keep going.

Let's say, I have some homework to do. In order to finish the homework, at some point I have to sit down at my desk and start working. And in my experience, actually starting is the hardest part, because after that I have few problems with continuing to work. And the process of "sitting down, opening the relevant programs and documents and starting to work" is not difficult per se, at least physically. In a simplified form, the steps necessary to complete my homework assignment are:

1. Open relevant documents/books, get out pen and paper etc.
2. Start working and don't stop working.

I know what I need to do, I just don't know what I need to do, so I can do what I need to do.

Considering how much trouble I have getting to the point where I can do step one (sometimes I falter between steps one and two), there must be at least one necessary step zero before I am able to successfully complete steps one and two. And knowing steps one and two does not help very much, if I don't know how to get to a (mental) state where I can actually complete them.

A different analogy: I know how I can create a checkmate if I only have a rook and king, and my opponent only a king. But that doesn't help me if I don't know how to get to the point where only those pieces are left on the board.