I need to make a skull cake or something edible really which has to be 3D from all sides. What would be the best way to do this without the moulds? Because they are quite expensive and the skull needs to be medium sized.

Thank You!

I see a LOT of carving in your future if you want a 3-D skull without a mold.  Yes, it can be done, but the molds make life a LOT easier.   I was just heading to my storage to retrieve my Wilton 3-D skull pan (now discontinued) and make some skull cakes.  Easy peasy.  Nordic Ware also makes a 3-D Skull pan:

http://www.amazon.com/Nordic-Ware-88448-Haunted-Skull/dp/B00MBPODXC

IF you absolutely cannot afford a skull pan, then I suggest you start with two, 8" round x 2" high cake layers. 

Put buttercream in the center and create a 4" high, 8" round tier.

Carve (a tiny bit at a time!) until you get a skull-ish shape that can stand on it's side, or at an angle.   (You can cut the bottom of the tier at an angle similar to the Wilton and Nordic Ware 3-D pans to get an upright skull.)

I used the Wilton oval pans (I think they were 9"x2") to make this 2-D Dia de los Muertos which was carved.

https://christinascakes.shutterfly.com/pictures/258  

https://christinascakes.shutterfly.com/pictures/258#256  

I used the Wilton 3-D skull pan to make this cake.  The difference is extreme.

https://christinascakes.shutterfly.com/pictures/346  

https://christinascakes.shutterfly.com/pictures/346#358  

I was very happy with both styles of cakes because they were exactly what I wanted for THAT cake.