8 bit adder with 2 CLA
Get a sheet of grid paper, the bigger the better. Draw out the waveform for all possible inputs of a 4-bit cla and then work through it and calculate the outputs. Write the decimal equivalent on there so it makes more sense. A 4-bit cla has a carry in and a carry out signal. Find the signal that you need to add from the first 4-bit cla when it is "overflowing" to the second 4-bit cla.

This is how I did this problem when I needed to solve it.
If you don't like paper then use modelsim.