Moving wordpress from TLD to sub-folder (URL structure only)

My recommendation is to use one WP installation and to simply add /blog/ to your permalink structure. This will give all of your blog posts a mytld.com/blog/postname format and pages will not pickup the /blog/ prefix.

Adjust Permalink settings at Settings > Permalinks and select the Custom Structure radio button. Enter /blog/%postname%/ in the field and save changes.

Your category and tag archives will pick this up automatically as well. Do not enter a prefix here unless you want additional layers in your URLs.