You can either start from a Visual Studio project template for AutoCAD plugin (see this tutorial or this template sample) or start from scratch.
In the second case,
- in Visual Studio, create a new class library project targeting .NET Framework
- add references to AutoCAD libraries (accoremgd.dll, acdbmgd.dll, acmgd.dll setting their Copy Locale property to False
In any case,
- add a new item of type UserControl (named "PaletteTab") with some controls:
- Label (lblLayer)
- ComboBox (cbxLayer)
- Label (lblRadius)
- TextBox (txtRadius)
- Button (btnRadius)
- Button (btnOk)
- add event handlers to some controls:
- txtRadius_TextChanged to txtRadius
- btnRadius_Click to btnRadius
- btnOk_Click to btnOk
- add the CustomPaletteSet and Commands class