How to access richtextbox via different thread - vb.net

Question

Hey guys my question is how can I access (update/read) richtextbox in a thread.
I just created a very simple code for you to understand what I am doing. I searched some articles on internet mentioned about invoke, delegate or backgroundworker, hope someone can come and tell me which and how to use. Really thanks.
Imports System.Threading
Public Class form1
Dim flag As Boolean = True
Dim startbtn As Thread
Dim stopbtn As Thread
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
startbtn = New Thread(AddressOf startfuction)
startbtn.Start()
End Sub
Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
stopbtn = New Thread(AddressOf stopfunction)
stopbtn.Start()
End Sub
'************** thread 1
Private Sub startfuction()
flag = True
While flag = True
richtextbox1.text = "Your process started" 'error
End While
End Sub
'************** thread 2
Private Sub stopfunction()
flag = False
startbtn.Abort()
MsgBox("You ended the process")
End Sub
End Class

Answer 1

Imports System.Threading
Public Class Form1
Dim flag As Boolean = True
Dim startbtn As Thread
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
startbtn = New Thread(AddressOf startfuction)
startbtn.Start()
End Sub
Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
flag = False
MsgBox("You ended the process")
End Sub
'************** thread 1
Private Sub startfuction()
flag = True
While flag = True
Me.Invoke(Sub() RichTextBox1.Text = "Your process started") 'error
End While
Me.Invoke(Sub() RichTextBox1.Text = "Your process stopped") 'error
End Sub
End Class
EDIT 1
Also when running threads, When you goto close your app you could run into problems because your threads will end prematurely..
do something like
Private Sub Form1_FormClosing(sender As Object, e As System.Windows.Forms.FormClosingEventArgs) Handles Me.FormClosing
flag = False
Application.DoEvents()
End Sub