Unfortunately there is no short and sweet answer to your question. Which I take to mean roughly
"how does one separate the collection of frequencies one would measure to gather the individual measurements and account for the different redshift adjustments."
First off I'll post the article myself and other PF members wrote to cover each aspect of redshift and cosmic distance ladder.
https://www.physicsforums.com/showpost.php?p=4370154&postcount=10this article is a slide show however it covers at page 167 apparent magnitude and spectrograph relations.
http://terrytao.files.wordpress.com/2010/10/cosmic-distance-ladder.pdf
the page referenced above is better defined here
http://en.wikipedia.org/wiki/Hertzsprung–Russell_diagram
the key point is when we gather a collection of varying frequencies in measurements there are numerous steps and processes needed to go through.
The use of Cepheid (standard candles) are extremely important as many of their properties are well known so we can eliminate many of those steps described in the cosmic distance ladder slide.
this site has a decent coverage and lab of each aspect of the cosmic distance ladder and the spectrogram usage.
http://astro.unl.edu/naap/distance/distance.html
hope this answers your questions or at least helps in defining your specific aspect your looking for
edit: as far as this being done through a gravitational lens relies on understanding the distortions caused by the lens
A key point to remember is due to no one method being reliable, scientist rely upon a database of previously collected results and methodologies