Based on the example from MSDN forum:
Making Specific Lines Bold in a RichTextBox- VB.NET[
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RichTextBox1.SelectionStart = RichTextBox1.GetFirstCharIndexFromLine(0)
RichTextBox1.SelectionLength = RichTextBox1.Lines(0).Length
RichTextBox1.SelectionFont = New Font("Thoma", 16, FontStyle.Regular)
RichTextBox1.SelectionStart = RichTextBox1.GetFirstCharIndexFromLine(1)
RichTextBox1.SelectionLength = RichTextBox1.Lines(1).Length
RichTextBox1.SelectionFont = New Font("Arial", 14, FontStyle.Bold)
RichTextBox1.SelectionStart = RichTextBox1.GetFirstCharIndexFromLine(2)
RichTextBox1.SelectionLength = RichTextBox1.Lines(2).Length
RichTextBox1.SelectionFont = New Font("Calibiri", 12, FontStyle.Italic)
[EDIT]
Above code can be "converted" into procedure. See:
Programatically Add Formatted Font To Richtextbox - VB.NET | Dream.In.Code[
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Good luck!