In the same way as you do it manually: by selecting range (single character or word or paragraph or set of characters, words or paragraphs) and pushing
Bold
command from ribbon.
Have a look here:
How to: Programmatically Define and Select Ranges in Documents[
^]
How to: Programmatically Format Text in Documents[
^]
Range.Bold[
^] property ;)
Note: if you save a document as a text, you'll lose formatting.
Tip: Run MS Word application. When new document is created, type some text, then start recording macro. Select text and bold it. Stop recording. Push ALT+F11 to switch to CodePane.
Here is what i get:
Sub Macro1()
Selection.MoveUp Unit:=wdLine, Count:=1, Extend:=wdExtend
Selection.MoveLeft Unit:=wdCharacter, Count:=7, Extend:=wdExtend
Selection.Font.Bold = wdToggle
End Sub