Research before you begin writing. There may still be times during the writing process when you need to do additional research, but having the bulk of the research done before you begin can help keep you on track. For many writers, a short trip to the Internet for simple research can lead to procrastination and wandering off on search tangents that have nothing to do with your book, which can stall the writing process.
Hang a calender in front of you and mark the days you'll be devoting to writing. Hang a note to yourself with the calender to remind you of the goal you're aiming for. Write five pages a day for sixty days to achieve your goal of a 300-page book in 60 days. Staying with a page goal rather than a time goal will keep you focused. Regardless of how long it takes, do the five pages a day.
Edit as you go. Read the previous day's work before you begin writing the current day's work. Make any edits or rewrites you have, then complete five new pages, which you will in turn edit the next day. This will help ensure that your work is finished when you complete your last page on your last day.
Set your book aside at the end of the 60 days. Your 300-page book has been written in your 60-day time frame, but you're still going to want to read it in its entirety before sending it out for publication.