I have tried making this before with disastrous results. I had this entire bottle of generic Metamucil just staring at me, tauting me really, in the closet. So, we gave it another go.
First, we dissolved 1 tablespoon of Metaucil in 1 cup of water. Make sure it is in a rather large bowl. Then, place in the microwave for around 4 or 5 minutes. Watch it closely, as it bubbles up. If it overflows into your microwave it is not easy to clean out (trust me on this one....I know). Next, let it cool for about 1 min. Microwave again for 4 or 5 minutes. Follow this cycle 4 times. Let it cool (it will be HOT). It turns into the coolest slime. You can even cut it with scissors. This batch made the amount that you see in my daughters hands. Enough for one person to have a slimy time with!