My first experience was actually a result of the start of Iraqi Freedom. I hadn't touched the editor before, been mainly trying to finish Cold War Crisis (I found it ridiculously hard at the time, but addictive). But with the start of Iraqi Freedom I got an idea for a mission to take place on Desert Island.
Basically I spammed one of the coasts with US groups and APCs, and the rest of the island to the north of me with Soviets. I ran into the unit limit and thought I had completely broken the mission until I realized I could simply delete some Soviets to fix it. I made myself the leader of one squad, changed my unit to a the Steyr Aug soldier and gave myself a Search and Destroy waypoint on one sole Spetsnaz way up in the north of the map on that infamous hill, who I intended to be Saddam.
On my old PC I got maybe 10 FPS max in this mission, but I played through it so many times, each time was different. I'd substitute myself as different things, like an Apache, or an A10, or sometimes I'd try it as the Soviets. Having such a massive battle going on around me was amazing to me at the time. Sometimes I'd just sprint right through it all and see how long I could last. I actually saved the mission as "The Hand of Justice" or something to that effect and it remained there in my SP missions for years to come... Until I lost it...
So that was my first ever editing moment. From then on I knew I was born to make missions.