I wrote my first inventory by making categories: anger (because anger is my trademark haha), high school memories, college, marriage, other, and good things (even though there was some good mixed in those other categories too). Then, I just wrote down everything I could think of for each category. I didn't worry about making it perfect or analyzing each memory, I simply wrote the memories.
After writing everything I could think of, then I filled in the blanks. Under each circumstance, I made subcategories:
Why: Why did I do that? What was I feeling?
Self-examination: In the book, it says for this section to analyze how your character weaknesses or strengths affected the situation. Then it asks if you see any evidence of pride, self-pity, self-deception, or self-will. In this self-examination section, I did write about how my pride, self-pity, self-deception, or self-will showed itself. I also wrote things like "I feared man more than God." And if there were some type of good effects or a part-good thing in a bad happening, I wrote about that. There is a sample list of questions you could ask yourself for the self-examination part in this chapter of the book.
Something that is crucial is prayer. Before every time that I wrote, I prayed that God would help me see the things He wanted me to see. I know there is more that I could have written, but at the time I was writing God showed me the most important things that I needed to be aware of. He even helped re-surface memories that I had forgotten about or pushed to the back of my mind. God also helped comfort me as I went through this process.
I went about my second inventory a different way. When I prayed to first start it, this is just what I felt strongly that I needed to do. Because emotion has been such a huge part of my life, my starting categories to write memories from were emotions or character traits from emotion: sadness, anger, regret, fear, bitterness, resentment, pride, self-will, self-pity, impatience, and misc. I had happy on there, but as I was writing, it didn't seem to work out somehow, so all or most of my "good" things about myself are in the misc category. Then, after writing memories, I went through a similar process of analysis. Some of them, I analyzed as I went because I've just been in an analyzing mode I guess.
I'd encourage you to pray as you go through your inventory process. God will help you see what you need to see, and He will comfort you when you need comfort.
Also, after you finish your inventory, move on to step 5 right away. Don't let this sit with you.
Step 5 is confession. Yes, you confess your inventory to someone. But it's someone who you trust very much. I ended up confessing mine to my husband. For me, I couldn't think of anyone better, especially because he did his confession of his inventory to me. For us, that's what works and is important in our marriage (having that open communication), and it brought us closer together.
As you are writing your inventory, don't worry about the fact that you will confess it later. Just write. It doesn't need to be perfect or impressive. Just do it. Let God guide you and help you, and you will be fine.