How to mock receiver in method

[forjoin92]

Here is my codes:
type A interface {
aaa() string
cstring() string
}

type B struct {
c string
}

func (b *B) aaa() string {
...
s := b.cstring()
...
}

func (b *B) cstring() string {
return b.c
}

How can I mock b.cstring() in B.aaa()?