An overhead cascade is juggling with your arms up in the air above your head & looking up from underneath the pattern. This can be done standing up but is easier to learn lying down on the floor on your back because you don't have to worry about balance & it's easier on your backbone.
Lie down on the floor with one ball. Put your arms up in the air with your palms facing the sky/ceiling, you will be looking at the back of your hands. Throw the ball to the other hand with the same arm motion you would use to do a press up or a bench press, it's a bit like throwing a shotput. Throw the ball back & forth. If you miss the catch, roll over, don't just close your eyes!
Try to reduce the amount of upper arm movement. Keep your upper arms pointing straight up from the floor & use your forearms like levers to make the throws as this requires a lot less effort.
Try to catch & throw using your thumb, fore & middle fingers only. Because your wrists are bent back, your weaker ring & little fingers will naturally start to curl inwards & may affect your throws.
Start with one ball, then work up to two & then three just as you did for the regular cascade.
Because you are underneath the pattern it is a little more difficult to judge the height to which a ball travels. You can try juggling either below or above your eyeline, that is make the throws & catches above your chest or over the floor above your head. You will then be looking at the pattern at an angle & you will be able to judge the height more clearly.
When you can do it lying down try it whilst kneeling & then standing up. Lean right back with your legs bent, look straight up & juggle. It is exactly the same but you also have to think about your balance.
Juggle a regular cascade, throw one ball high, catch it above your head & continue with an overhead cascade, throw another high one to return to normal juggling. Try going into an overhead cascade from a Three ball flash, or a Behind the back throw, or a Rainbow Cross. Also try other tricks over the head, Over the top tricks, Columns & Mills Mess are all very possible.