hi, does anyone have a guide on how to set up invoicing module? i.e. i would like the ability to customize the invoice template to make it look nice, and also test the ability to send out the invoice to customers

I don't have a guide for you but you can create custom templates under Customization > Forms. You then need to apply the template to your desired forms. You can try your hand at NetSuite's editor but a basic understanding of HTML helps in working in the source code.