I look them up in Unicode when I only need a few.
If there is a lot of text to type, you can change the input mode on the keyboard to be Greek and then just type letters. The videos I watched about it didn't make it sound particularly easy, so there's probably a better way. Mac users have a character viewer that they can turn on.When you go to Insert > Symbol, there is a subset of symbols called "Greek and Coptic" that can get you there quickly. It provides the full range of upper and lower case Greek letters, but you have to click each symbol one at a time. Word allows you to use a symbol palette for the Greek letters. One trick in Windows (or Mac) is to create the content in Word and then copy/paste it into Canvas.As you notice, there are several missing and most of those are probably more useful for mathematics than general Greek conversation. The supported characters are α ß Γ π Σ σ µ τ Φ Θ Ω δ (alt+224 through alt+235). For example, to get a α, you hold down the alternate key, type 224 on the numeric keypad, and release the alternate key. In Windows, there are alternate codes for many of the Greek letters.
There are several solutions available, but almost all of them involve modifying the question from how to enter Greek letters in Canvas to how to enter Greek letters in Windows (or Mac OS). This question comes up periodically with international characters - those teaching Spanish or French often want a way to enter accented characters. Canvas supports Unicode with an UTF8 encoding, so the characters are available in the same font as everything else, you just need to be able to enter them. Would not use the equation editor for this unless you need a Greek letter within a mathematical context.