HELP: Reading old Greek script

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After a couple months' study, I'm done with the Greek alphabet and can read anything printed, also write many words...or I thought so until an hour ago, when I visited the Khora Monastry/Kâriye Mosque (pr. a museum).

I realized that more than half of the script on the walls and eikons is still unreadable to me due to very different shapes of letters, ambiguities between some of them and sme weirdly alien shapes.~:mecry:

How can I improve my reading? Are there any free/affordable online/offline guides?

:help:
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