Doing my first 3-D cake and having problems on how to put the head made out of RKT . If anyone has any ideas or know how i can go about and doing a good job and not having the head fall off the cake. I would really appreciate it thanks

IDK exactly what shape you're making, but have the head on a board (cut to size just like you would a cake), supports in the cake below, place the head on, drive a long dowel (i like bamboo skewers) or two all the way through into the base.

Form the head around a dowel (like the Wilton hidden dowels) then after the RKT is set up, you can twist the dowel to remove it. Insert the dowel into your cake and cut it so that it extend a little above the cake. Then you can place the head on the dowel. That's how both the baby's head in the toy box cake and the skull on the pirate skeleton cake are done if you look in my photos