You stand at the correct distance so that you can see the reflection of the secondary just fractionally bigger than the actual secondary Ken.
To do this correctly you would use a card with a peep hole in it mounted on a tripod for instance, & keep positioning the card until the exact position & distance is attained. It's on the internet, & this method is used to roughly align after stripping the scope for a corrector or secondary replacement etc.
Edit: Adding article
http://www.mira.org/ascc/pages/lectures/collim.htm