The below steps help you to create a sample Dynamics GP customization with using Modifier and VBA.
1. Open a window in Dynamics GP which you want to modify/custoimize.
2. Go to Tools–>Customize–>Modify Current window(Ctrl+F10)
3. Then modify this window in Modifier with adding your custom fields.
4. Return to Dynamics GP (File–>Microsoft Dynamics GP).
5. Now change window type form standard Transaction Entry window to current Modified window (Tools–>Setup–>System–>Alternate/Modified Windows and Reports).
6. Now open the modified window(You can see the custom fields from this window).
7. Add this window to VBA Editor(Tools–>Customize–>Add Current window to Visual Basic (Ctrl+F11)).
8. Add the fields to Visual Basic (Tools–>Customize–>Add fields to Visual Basic (Shift+F11)).
9. Click the custom fields whatever you want to use your customization.
10. Now go to the VBA editor(Tools–>Customize–>VBA Editor(Alt+F11)) and write the code to your custom fields.
11. Save and build the project and return to GP and test your customization.
Hope this helps!!!