"Guessing" as I've not typed it in to check:

Declare variables as "public" in say frm1.  Then to pass something from frm2
to frm1:  frm1.myVar = 42

Well done for asking this, it is a tricky one for new programmers to .NET,
and I don't think Microsoft have ever really articulated how they thought
this "should" be done.

By default a .NET program will shut down if the first form closes, so what I
do is create an instance of the second form from the first form. You can then
either hand the variables through the constructor for the second form, or
simply pass them as Public variables.

The following code shows both methods. Make a new windows form project with
two forms, each with a button, and add the following code:

Public Class Form1

   Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles Button1.Click
       Using f As New Form2("Hello")
           f.PublicVar = "World"
           f.ShowDialog()
       End Using
   End Sub
End Class

Public Class Form2

   Public PublicVar As String

   Private mstrVar1 As String

   Sub New(ByVal var1 As String)

       ' This call is required by the Windows Form Designer.
       InitializeComponent()

       ' Add any initialization after the InitializeComponent() call.
       mstrVar1 = var1
   End Sub

   Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles Button1.Click
       MsgBox("mstrvar1=" & mstrVar1 & "; PublicVar=" & Me.PublicVar)
   End Sub
End Class