In present day, there are lot of circuits seen designed on flexible pcb. Looks like the trend is becoming popular day by day. Flexible PCB is veru much demanded in designing wearable product these days.
I use Eagle to make pcb. I know everything required to know for designing 2 layer rigid PCB. But I am planning to start designing flexible pcb as well. Searching in the internet I didn't find any appropriate tutorial, points to design flexible pcb.
If you have any tutorial materials or link for designing flexible pcb, please post it here. I think it will be helpful for others as well.
Altium Designer seems to have a focus on flex PCB design, you can check it out.