You have an outline and you're ready to write! Awesome!
There is no specific way to start writing a book. You have to do what you feel comfortable with. This is your book, just make sure that you are writing your book for you.
1. Program
When writing, it can be hard to focus. There are many programs out there, and you will only know which one you like the best if you try them.
Wattpad
-Easy to write on
-Saves work often
-Efficient if you plan to publish on Wattpad
-Can be distracting
-Have to exit out of each individual chapter to get to another
Google Docs
-Great for editing
-Saves work constantly and no need to back it up (it does it for you)
-Ability to change the font and anything else(use Comic Sans when writing and switch after)
-Can be distracting
-Able to access the entire document and jump to specific chapters easily
Writing in a Notebook
-Not distracting
-Easy to take with you anywhere
-Easily customizable
-Can't organize as well
-No copy and paste
-Can be damaged/lost
2. Order
Your chapters do not need to be written in order. If you want to write your chapters all backwards because that's how you get the inspiration, do it. Many people get stuck when writing simply because they don't know how to bridge the gap between the chapter they have just written and the chapter they want to write.
3. Criticism
Keep in mind that there is no improvement without criticism. If you are too sensitive to accept criticism, I wouldn't suggest publishing your work. Some people want to help, and some people just want to tear you down or have regard for your feelings. Learn to differentiate the two and only keep the helpful opinions in mind(whether positive or negative).
Helpful Criticism:
-helpful
-honest
-not always positive
-gives you advice or specific things that you can improve on
Unhelpful Criticism
-can be both positive and negative
-either compliments or rejects your work without any ways to improve
-honest, but not always logical (personal opinions)
DO NOT ALWAYS BELIEVE UNHELPFUL CRITICISM. IT WILL EITHER HURT YOUR FEELINGS OR BOOST YOUR EGO AND IS MOST LIKELY WRITTEN BY SOMEONE WHO DOESN'T KNOW WHAT THEY ARE TALKING ABOUT.
JUST KEEP WRITING!