I want to make a cd4049 cmos inverter spice model
The precise method depends on the particular spice engine.

Most have something called a subciruit or package definition that would let your create the component as a schematic. 

Many also have the ability to define the behavior of an element programmatically.
By entering a formula or script, relating the various values.

There are many different levels of FET transistor models.
It is beyond the scope of this answer to describe them.
A large number of them are described in this HSPICE manual,
the manual of your own particular spice implementation will likely has similar descriptions of the models it supports.

To determine reasonable values for the parameters, you are going to need to look at the datasheet for the component, which you should be able to download from the manufacturer.

You may in-fact be able to download a existing spice model for the component from the manufacturer -- or from the website of a spice engine -- though your might have to convert it to the format preferred by your spice engine.