I cant help but notice when reading older examples of English text. The spelling of words are very very different from our contemporary English words. In most cases there's no fixed spelling for a word, yet today we have a standard of spelling for every word we use. How did this come about?
The simple answer is when dictionaries became popular.
The less simple answer is that English spelling isn't standardised. There are huge differences between British and American English spelling.
Unlike the Romantic languages, for instance, there is no council to formalise English spelling, so words are basically spelled the way they're generally spelled by most people. Even within British English you see multiple spellings for the same word - inquiry/enquiry for example.
In the days before the printing press, books were manually produced by copyhouses. There was variation in spelling in accordance with local pronunciation and usage, but each copyhouse kept very consistent spelling in the books that it produced.
This all fell to pieces with the printing press. Suddenly the same word was spelled in any of the regional variants, even in the same document. In many cases, the compositor picked the spelling that made the printed columns line up nicely.
Eventually spelling became standardized, but not in any logical manner. That's why we get mixed up words like "busy" where the spelling comes from Wessex, but the pronunciation of East Anglia.
History of English Spelling: D.G. Scragg.
English and How it Got that Way: Bill Bryson.