Hi,
There are some good examples of the CPU design on the network that you can look at to understand how a simple CPU code works.
For instruction, you just write a VHDL code for each and every one of them. Depending on the type of instructions, you can choose different ways of writing your code. The easiest one is to create a case statement for each instruction and then write the code there, the other way (a bit more complicated) is to write micro code and then use the 5 bits of the data to be the starting points for the instrucitons in your microcode memory.
When you are done with your coding, you use Modelsim to simulate your design and you will get the timing information for each instruction.
Search internet for "simple 8 bit CPU VHDL" and you will find some good sites with information,
Cheers,
/Farhad