Hey Roland --
Actually, freelance is great once you get the hang of the invoicing process. However, I agree with you in that until you figure things out, it can be pretty frustrating.
The IRPF that you would deduct depends on the epigrafe that you registered for when you registered as an independent worker and how long you have been registered under that epigrafe.
For example. The standard IRPF for freelance English teachers is 18 or 19% of the invoiced amount(before taxes). However, for those who are newly starting out, there is a reduced IRPF of just 9%. After two years, you must start deducting the higher amount.
The IRPF is similar to the income tax in the USA. So, this percentage is DEDUCTED from all invoices that you send to clients. The client then becomes the responsible party to pay said amount to hacienda.
The IVA is similar to the USA sales tax. This percentage you ADD to all your invoices and then depending on how you do your taxes, you pay this amount to hacienda either monthly, quarterly or annually.
The benefit of billing IVA is that you can deduct any IVA that you pay (so keep a good record of your expenses).
English teaching alone is not subject to the IVA, so many freelance English teachers find themselves in a quandry when they have to pay IVA for their supplies, but cannot deduct it anywhere.
I am not certain of the IRPF amount, nor IVA amount for web design.
If you have any other questions, feel free to give us a buzz!