I am submitting the Rental Tax on 210 form myself. On the internet there is an ENGLISH version information/instruction sheet to print out,
Type in Google ;
"INFORMATION ON TAX DECLARATION ON FORM 210FOR NON RESIDENT OWNERS OF ONE PROPERTY"
This sheet will help you to fill in the 210 form. You need one Form for each Owner of the property!!!
You also need to call in at your Agencia Tributaria to get some Stickers (Etiquetas) with your NI numbers on, together with a 210 form for each person on the property ownership and also complete a form 030 to tell them you are doing your own Tax return, which if you have rental income from letting the property you have to put the relevant information on that 030 form!
All sounds hard but with patience and some initial work it saves you having to pay a fiscal representative. Some Fiscal representatives however, only charge a reasonable fee eg 60 or 70 Euros for two owner names. Many are advertising on the internet alongside the 210 FORM web page.
It's a choice you have to make!
Ronaldo