Any top tips for science project crystal growing?

My 10 year old got a crystal growing kit for her birthday. We’ve had one attempt and it turned out a dish of stuff that looked like a purple frozen puddle – sort of icy and watery. The kit was this one
and the instructions inadequate for us two amateurs. The bits we had a problem with were the fact that we were cooking it for about an hour and all the crystals still hadn’t dissolved. We got worried that it was overheating so decided to stop and move on to the next step. But then it wasn’t clear what to do with the mixture so we just put it in a plastic pot supplied in the kit. Should the crystals just have grown from there or was there something else we should have done with the mixture? Or was it that the mixture wasn’t prepared right. Where should we have put it – would a hot radiator have been too hot? Should the pot have been covered?