Let you have project with two forms (Form1 and Form2) with command buttons on them (Command1 and Command2 respectively) with click event procedures (Command1_Click() and Command2_Click()). If you want from Command1_Click() (on Form1) to call Command2_Click() (on Form2), you can write:

Private Sub Command1_Click()

    Form2.Command2_Click
End Sub

The only thing you need to do is to change Private modifier of Command2_Click() to Public in order to be visible outside of Form2.