Suppose we have a content type called "Procedure Document" with following columns:
1. Document Category
2. Reviewer
3. Review Date
you can have multiple columns but above columns are just for testing purpose to show you the idea.
Once you have uploaded a document and supplied the metadata as shown below:
Now edit the document and follow below steps to add metadata into that document:
1. Go to Insert tab
2. Click on "Quick Part" > Document Property
3. Choose fields that you want to include in the document
Hope this helps.