Here are the steps we need to follow to place order through rest API for guest customer.
We have explained each steps with postman screenshots of  API request. we have used all URL of our local setup. Lets us know if any concern/issue in comments...

1. Create Admin Token

2. Create Cart/Quote for Guest Customer

3. Add OR update item to cart 

4. Remove Item From Cart

5. View Cart Item 

6. Add shipping Information to guest cart

7. Provide Payment method and Place Order

After above steps you can check the order is created in admin panel for guest customer.
Happy Coding !!