How to mock external methods for unit testing

I want to mock methods for testing in Go. I am unable to figure out any steps/pattern that is usually followed to mock.

Consider below code and help me out in understanding.

package fetchClient

client.go

type Client struct {
    URL     string
}

type ClientData struct {
    ID      string       
}

func (p *Client) GetURL(ID string) (*ClientData) {
    data := &ClientData{}
    //Some work to store client information into data struct and return it 
    return data
}

package fetchClientData

FetchData.go

// Function GetData that accepts a client ID as argument and returns client URL
func GetData(ID string) (*fetchClient.Client.URL) {
    param := (*Client).GetURL(ID) 
    return param
}

FetchData_test.go //Want to know how to mock as asked in below question.

Question: How to mock a fake GetURL function in FetchData_test.go file so that it does not call actual GetURL function in FetchData.go file, instead mock it.