By default the plugin looks in titles, content and excerpt as well. First you need to disable all these options on the General Options -> Sources panel. However in order for the plugin to work, at least one field must be enabled, therefore make sure that the Search in post (and CPT) IDs?option is enabled.
Step 1: Make sure you have the correct post types enabled
Step 2: Disable everything, but search in CPT IDs
This step is to make sure that at least on field is enabled, otherwise the plugin might skip the search process completely.
Step 3: Make sure the Generic front-end filters are unchecked or disabled
If you don’t need Front-end filters, use Option 1
..if you use Front-end filters, but not the generic ones, then Option 2
..if you want to be able to search titles and content, then Option 3
Just add keywords, and it’s done
On the post editor screen, keep adding additional keywords to the meta box (space separated), and the plugin will use those exclusively.