This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
Woz: The author of the report I received from you is wrong when he says no ROM of large size was assembled by hand. I could never afford an assembler for the 6502 which I bought for $20. First, I connected the 6502 to Static RAM and a video terminal of my own design. I then wrote a short "monitor" program to watch the keyboard and display characters, both under interrupt and polled. We didn't have 256 byte PROMS, just 256x4 PROMS at that time. I used two of the ones we burned for calculator development at HP. The first hardware bringup had a few frustrating hours but I got it working with polled keyboard that night. I then wrote a 256 byte "Monitor" program which watched the keyboard for hex data entry (address:data data data) and hex display and program initiation ("Run"). I got very good at typing in hex and very very good at checking data entry carefully, character by character. I still can't read a credit card number without a high expectation of mistake, but I have good habits to this day.
 
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