For us it was a discussion early on in our relationship, did we both want marriage? How many children were we thinking? What did we think our lives would be like? We were early 20s and just starting out in the world when we met.
There was no proposal as such, we had a rough timeline, talked a lot, house sat for my sister to live together for 2 weeks as we were both living at our parents' houses, then moved in together a few months later due to a job. We had a timeline for all of these things. In line with that and talking about what we wanted in life we then went and chose a ring together. At that point we were already technically engaged as we had agreed to marry. We just didn't tell anyone until we had the ring so we have an anniversary for it but neither of us even knows what that is!
We have been together 30 years. If you have already agreed you will marry each other you are already engaged.