If you've been doing regular writing practice for a while (see Top 6 Reasons To Do Writing Practice) you will probably start wanting to spend time working on specific craft skills like descriptions, dialogue, action scenes, etc. Good idea! Improving those skills makes your stories better.
So where do you start? There are lots of ways to work on your writing skills. Do a search for "writing exercises about XYZ" and you will find tons of ideas to get and keep you going. I thought I would share one of my favorite ways to decide what skill to focus on. Bonus--it involves reading!
First, pick out a favorite book or short story to re-read. If you choose a novel, you might want to go with a shorter one since it will be easier to analyze.
Next, write a few notes about what you remember about the book. Especially make notes about the parts you really loved (and the parts you weren't so thrilled with if there are any). If you have thoughts about why you loved or hated different parts, note that too.
Now sit down to read, but keep your notebook and pen handy. When you come across something that is really wonderful write it down. (If it's long, paraphrase if you like). Make a note of what category of writing it is (description, line of dialogue, etc.) and why you like it. Don't forget to write down the page number so you can go back to it later.
Once you've gone through the story making notes about your favorite bits, write up your overall impressions of the book. Give extra attention to what you liked the most about it and why. Then go through your notes to see if there's one writing area that shows up most frequently. Maybe you love the dialogue in this story, or maybe the descriptions thrill you to your toes.
Whatever area really stood out for you in this book, pick that as your craft skill to work on. Find some exercises that appeal to you, and make a plan to work on them for a few weeks until you're feeling happy with your skills in that area. Don't try to emulate the author in your writing, but do think about how that author might write the exercise and what parts of that you would like to incorporate.
You can also do this by picking an area to work on first and then examining a favorite story for great examples of that skill. For example, if you know you want to hone your dialogue skills, seek out a book that you remember having great dialogue and read through it to find your favorite examples. Analyze what makes you like them, then go to your writing exercises and work on putting those qualities into your own writing.
Of course you can pick an area to work on and do exercises without doing the analysis beforehand, but I find that doing this helps me focus on things I especially want to try to bring into my own writing. I hope it helps your writing, too.