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During the pilot, a service's particular error or alternate login page will have an additional button called "Try authenticating via Touchstone". So for example, if you try to log in to Stellar and your certificate does not work or you do not have one, you currently get the page below. 
When the Touchstone pilot goes live, there will be an additional section underneath the current MIT Community Users section on the page below. I pulled an example screenshot from the stellar-dev.mit.edu web site. (See second attachment.) I'll try to put this in the wiki later today.
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