PRGs exist if computationally secret encryption schemes exist
I don't know of any direct way of doing this. In particular, in contrast to the way most encryption schemes work in practice, in theory there is no requirement that the output of an encryption scheme look random at all. The only way that I know to do this is to prove that computationally secret encryption implies one-way functions (not hard to show, but not completely trivial, since it is only true when the amount being encrypted is larger than the key size). Then, using HILL, one can construct a PRG from any one-way function.