First step is to do proper keyword research to choose the 300 keywords that you will build your websites around, If I were you I would have chosen keywords with very weak competition in the first page of Google to increase the possibility of ranking the websites on their own and cut the backlinking efforts. You can use "stealth keyword" software for keyword researching that amount of keywords, find it in the BST section, it will make your life easier.
2nd: Purchase partial match or exact match domains related to the keywords that you found.
3rd: Buy a private hosting space with different ips and host 5:10 site on each IP.
4th: Use an easy CMS preferably wordpress to install on all of the 300 domains, to make things easier use a wordpress management service like worpit to keep it easy to manage all of the websites.
4th: Find an agency or a couple of freelancers to write the content around the keywords that you previously researched. 5 articles per/site is a good start, then you can create more later for the best performing websites.
5th: Post the content and optimize titles and metal (on page SEO) for each article and website.
6th: Index the 300 hundred websites by using an indexing service or building a couple of backlinks for each website (automated links for sure)
7th: Leave them for 2-3 months then register in a SERP tracking service to check each website current ranking to determine which websites need small amount of backlinks and which will need more links.
8th: Start building backlinks by a good tool - can't provide good info about the type of backlinking strategy that you should use as I never ranked that much of websites before)
9th: Keep us informed how this goes as many members including me will be interested to see if ranking that much of websites is profitable and worth the time, effort and investment or not.
Best of luck.