If you have a static site and want to change to a wordpress blog then the best way is to follow this simple plan.

Let's say your static site consist of the following:

index.html
about.html
contact.html
order.html

rename your index.html to index_old.html then install wordpress and start adding content to the blog. On your current html pages you will want to make sure the home links go to your domain dot com and not index.html so edit those.

after content is on your blog then you may want to edit and add some more links in your static html pages leading to the content on your blog. Other than that you are not going to change anything else. This will not effect any current rankings at all.

keep adding fresh content to the blog every day or every other day and make sure you social bookmark those post. After a few weeks you will see a huge increase in rankings (assuming you are doing your keyword research proper and posting seo'd content properly).