The are so many challenges to being a writer and for each of us the biggest challenge could be different. For me personally I would say the most difficult part of actually writing is trying not to use a passive voice. Making the words work harder for me and not letting them just sit on the page is one part of the writing process that I deal with at every stage.
One of the first things I noticed when I started learning about the passive voice was that a passive sentence is generally wordier than an active voice. I tend to be wordy. Part of that is from doing Nanowrimo probably. When you consistently take part in a challenge that is based on a word count it becomes your driving force. My stories are usually right around the fifty thousand word mark anyway so if they fall short I go back to add words here and there. Many times I will take a sentence, paragraph or scene and by adding the extra words take it from a working voice back into passive.
Then I am tasked as I go through editing to be able to fix a problem that I created. I already have trouble editing because I am still learning so many aspects of it as I go. I recently learned that there is even a mode on my computer to help me with editing. I also have been introduced to a few sites that will help me with grammar as I go through slowly editing the rough first draft.
I am quite a bit older than so many of those I see online taking part in Nano as well as other writing challenges so when I interact people assume I have been actively involved in the writing world since I was their age. If that were the case I would most likely be better versed in how to do much of what I am learning now. I always enjoyed the creative side of English classes in school but I hated the grammar, formatting and formality of the rest of the class so I never did well. I think it shocked many when it was a career I chose.
The best advice I have gotten so far when it comes to writing and looking for that subtle passive voice is to consider if the sentence, paragraph or scene would function the same or better without the words. Can you pare down and still keep the message the same? If so then it is better to use less and say more. That is what is meant by making the words work harder for you. You can Google examples which is what I did. It took awhile reading through them to truly understand how to fix the issues. I would look at the passive sentence and try to guess how they would fix it then scroll to see if I was right.
After more than a dozen samples I started to improve. It is much more difficult in my own writing to spot the errors. In the examples I knew that they were passive and in need of altering. With my own writing I have to read each sentence with a critical eye asking if it is indeed passive. I believe that for my current editing project I will be going through it three times before I do the first revision. Once for typos and glaring errors like missing words or plot wholes. Second I will be sending it through one of the grammar checking sites and once I have made note of those issues I will be looking at each section for passive voice to make every word work as hard as I do.