Learn German: Die Blume ist schön. - The flower is beautiful.

You would say Eine Blume ist schön. Because Blume is a feminine noun (which we know from the definite article die), it takes the feminine indefinite article eine for 'a' or 'an'.

Because Blume is a feminine noun, shouldn't the adjective change to schöne?

No, because of where it is in the sentence. When an adjective comes after the verb (like ist) to describe a noun, it does not get any special endings. It just stays in its basic dictionary form: schön. You would only add an ending if it came directly before the noun, like in die schöne Blume (the beautiful flower).

What should I do if I can't type the ö in schön on my keyboard?

If you cannot type an umlaut (the two dots), the standard rule in German is to add an e after the vowel. So, you can write schoen. Be careful not to just drop the dots and write schon, because schon is a completely different word that means 'already'!

Is ist a regular verb here?