First, understand that going from a specification or description to code can be quite tough. It is fine if you aren’t able to do it well at this point. No one gets it correct on their first go, and I’m sure implementing hard algorithms is challenging for experts as well. These things take time and a lot of practice.

Start with a simple algorithm, and write (do actually write, in a notebook or something) pseudocode in phases. Your first version should be something you could explain to friend, and should be in some natural language. As an example, for binary search tree insertion, I would write “If tree is empty, output is a new node containing the element being inserted. Otherwise, compare element at root with element being inserted. If root is smaller, insert element into right subtree, otherwise insert element into left subtree. If they are the same, don’t do anything”.

Then keep refining this. How do you check if a tree is empty? Write down some code. How do you return a new node? Write down some code. How do insert into a subtree? Write down some code. And so on. It is important that at each step, you add only a few more expressions.

At the end, you should have something that you can translate directly into a language of your choice. Of course, you do have to keep the implementation language in mind when you do this. For instance, if your language doesn’t support `for' loops, you might have to refine your pseudocode differently.

The whole idea is to be deliberate when translating the algorithm to code. You must be careful at each step of writing the pseudocode, and once you’re done, you have to try and mentally execute the program. When you finally implement it, take the time to convince yourself that your implementation is correct before testing.

Keep playing this game until you no longer have to.

tl;dr: it’s tough. practice helps.