Tips for making my own yogurt?

My mom makes yogurt all the time. Maybe your slow-cooker is getting it to hot or letting it dip to cool. My mom has done it two ways. You prepare it just the same way you would for the slow-cooker way, but then instead of putting it in the slow-cooker, you put it in jars and either set them on a heating pad or put it in a sink full of hot water. You need to monitor the temperature, making sure it stays in a specific range (sorry, I can’t remember what that range is!).
As for the runny-ness, homemade yogurt tends to be runnier than store-bought. It’s really good over granola or cold cereal.
I know this probably wasn’t was you were looking for, but I hope I helped somehow.

In the summer (if you don’t have air conditioning) a kitchen counter should be warm enough for the yogurt, a slow cooker is overkill. Back when ovens had pilot lights, most home yogurt makers made yogurt in the oven without turning it on. Just the pilot light made it warm enough, even in the winter. Be very diligent about cleaning everything first, including your hands. Stray germs of the wrong type will ruin yogurt. Use very fresh milk; it’s best to make yogurt the day you get your milk.

You want to have the temp between 100 degrees and 115 degrees F (110F is ideal). If you are not using a yogurt maker, then use a thermometer to keep track – too low and it won’t “yogurt”, too hot and you kill the culture. I use a meat thermometer that has a long cable going from a probe (which is set next to the jars of cultured milk) to the digital readout.

I prefer to use a yogurt maker, but it only does 1 quart at a time. If I need to make more than that, the best setup I’ve found is to use a “trouble light” with a 100W bulb in my oven. The cord keeps the oven door open a little so it doesn’t get too hot, and I drape a towel over the opening if it doesn’t heat up enough. Test it out for a few hours before going “live” to see what you need to do to get the temp steady in the right range.

If I use cow’s milk it thickens up just fine. Goat milk is a lot thinner, but tastes the same. I culture mine for 24 hours, longer than most people like, but it does make a nice tart yogurt that has all the lactose used up.