Step 2, add a blurry blob of darker, greyer blue where the ice is the deepest
Step 3, color the lineart darker blue on the outside and lighter blue on the inside
Step 4, duplicate the lineart and blur the shit out of the one on the bottom. Make it a clipping layer.
Step 5, use a lighter, more saturated blue to light up each side of the cube like it's a simple flat surface. Merge all the ice layers.
Step 6 (OPTIONAL), find a texture or a brush similar to this one and put it on a screen or dodge clipping layer over your whole ice. Lower the opacity until it looks good.
Step 7, add bright shines with a white brush. Ta da. Ice