There are lots of ways to do it automatically if you don’t fancy typing them in manually?
I ‘highlight quote’ mine most of the time. You may need “Enable quote reply for highlighted text” ticked on your Preferences/Interface page - but I highlight the part of the post I want to quote, and a little floating ‘QUOTE’ box appears. Tap that and it auto-fills the excerpt as a quote in your reply window. You can then either reply as normal within the same topic, or navigate to another and cut and paste it there, etc. 