My approach has always been to do it manually (though not for a blog since they can easily be exported/imported). I don't care how many pages there are, a few dozen to hundreds. I prefer to "touch" each page and give it a review.
Before you install WP Start by creating an outline of your existing site. This is your chance to toss what you don't want, to reorganize content into something better for users, SEO, or sales - whatever your goal is.
If SEO is important to you, create a keyword to content map if you don't already have one. Be sure to keep this close by when you begin to move content.
Create a redirect map. What pages will be redirected (301d) and where the user will land when they are redirected.
Install WP Get your WordPress framework installed (WP Core, blank pages, & navigation). I often build the WP version in a sub-directory like /temp or somesuch. You can lock the directory with server level access or if you're not worried about someone seeing the site you can simply change the privacy setting to keep the robots from indexing the site.
If you're savvy with WP Themes and PHP now would be a great time to optimize the theme for coding, analytics, etc.
Check the click paths (navigation) and be sure it's intuitive & user friendly.
Be sure you brand your 404 page (don't just use the default WP 404).
Add your content, SEO data, and in-page hyperlinks. Test the navigation and consistency of the messaging/voice. Use a link checker to find broken links.
Install your redirects (301). You can use a redirect plugin in like Redirection. Test your redirects.
Be sure robots.txt is in place and configured the way you want. You can find posts in the WP Forums with suggestions on what to include/exclude.
Switch Over Grab all your old site files and place them in a new sub-dir for safe-keeping just in case something goes sideways on you.
NOTE: the following is from memory - the detailed WP instructions are here: [codex.wordpress.org...]
Login to the WP admin. Go to settings and change the path to WP and the URL so they reflect where you intend to move WP to. Save changes but don't try to access the site yet. (It's not there yet).
FTP in and move the WP install to the new location. Then go to the Admin panel and refresh it. You should see the login page.
Lock down wp-login and /wp-admin (block access to these from all but your IP or IP block) Search for hardening WordPress on the WP site.
Be sure your analytics are in place and recording.