SEO BEST PRACTICES (Deleting 0ver 1300 articles)

Hello,
I am not sure if I can ask these kinds of questions on here but I will try anyway. I currently have a client that has a blog (www.website.com) with over 1400 articles. I am in the process of redoing their blog and will be keeping only 100 articles (Per my client's request). I would like to know what you recommend in order for me to respect the Google guidelines in terms of SEO "good practices". I have heard/read about the 301 redirects but I am not sure how to get this done quickly/properly in my case.

Any ideas/suggestions?

CURRENT BLOG: http://www.website.com
NEW DEV BLOG: http://www.website.com/demo

Thanks!