The way I was taught to launch a small boat, and this may well have changed as I'm feeling as old as Victor Meldrew so treat it as anecdotal rather than advice, was like this:
1. Reverse to the top of the slipway.
2. Secure a line between the trailer and the tow hitch.
3. Use the tow hitch as a belay post (if you know what I mean) and gently, slowly, let out the line so that the trailer gently moves down the slipway.
4. Once the boat is launched gently drive the towing vehicle forward with the line secure until the trailer is clear of the water/green slippy stuff and then brake the trailer.
5. Reverse back to the trailer and hitch on as normal.
6. Recovery is similar.
(the above assumes that you have a jockey wheel)
It may be more difficult to do this single handedly: and even with a crew of three I used to have real problems controlling the rate of descent at times!
Like I said, its the method I was taught years ago when Queen Victoria (God Bless 'er!) was a nipper and slipways were empty,deserted places, so the practice may not be recommended in todays crowded world.