I started out overkilling it.
1. Clean stuff out of the gap.
2. put down the backer rod (for me, this was mostly used to fill up the space).
3. used clear waterproof silicone caulk on the backer rod to attach to concrete sides (this was the overkill, I stopped doing this pretty quickly).
4. applied the self-leveling caulk. If you're doing any amount at all, I recommend buying the big (about 30 oz) tubes of the self-leveling caulk (instead of the smaller about 10 oz tubes). At my Lowes, the smaller tubes are about $8.50, while the bigger tubes are around $13.50, so (almost) triple the amount for less than half the price. You'll also need a bigger caulk gun. And you can go through a lot of this stuff if you aren't careful, hence using the backer rod stuff. Also, the backer rod will float, so ideally you'll shove that stuff in the gap and it will kind of be jammed in there.
5. I don't like the look of the self-leveling caulk, so I put some play sand (can get at Lowes) over it. Don't know how that will do over time.
Like I mentioned, if you're not careful you can go through a lot of that self-leveling caulk. When I started I was buying the smaller tubes ($8.50 a pop) and could use 2.5 to 3 tubes for about a 10-12 foot span. Ouch, considering I had a lot more than that to do. So, get the bigger tubes, use backer rod (you can even fill some of the gap with sand if you want, just leave enough space for the self-leveling caulk), and try not to be messy with it. Oh, and you might want to wear gloves or latex/nitrile gloves to keep the stuff off your hands.
*** edit to add ***
I used Sika Grey Self-Leveling Sealant. My guess is that it's pretty close to SL1 quality.