push the brushes fully inside the guides.
On some starters there is a hole in the guide that a pin goes in.
On Briggs they come with a U shaped piece of wire that pushes out when you push the armature through the bush,
I rebuilt a Denso a few week back. What I used was a small cotter key straighten out. I ran the cotter key through spring so the spring couldn't go flying and then installed one brush at a time till all four were installed. As I said I don't know what brand starter you have?