This is maybe the first question that comes up concerning learning to read classical Tibetan and possibly translate it, for personal use or for the public.
You need a first step and then a continuation taking more steps. Take just anything in Tibetan and try to see patterns. Find the same patterns in other parts of the text.
Look at existing translations, see how parts and the whole are translated. This is very similar to how programmers look at existing source code to understand how it was built and how to use the same elements for future coding purposes.