When I did my first renewal eighteen months ago (second renewal is coming up fast!), it was just a matter of
a) being in the same status as before (comparable kind of job), and
b) reading and filling out and filing all the paperwork per the instructions on the forms.
I did have a lawyer review the forms with me, and found no surprises.
The renewal took five and a half months during which time, on account of your having submitted the paperwork properly, your status is legally extended. You must carry a copy of the documents which you have filed, and a copy of the certified receipt of mailing (there is a specific official type of mail to use so that you get this proper receipt; I forget whether you can hand in the papers in person at a foreigner office in Spain and get something similar).
You asked when to start the process - any time within the 45 days prior to your current permits’ expiration dates. In theory, you could do it within a couple of months *after* the expiration date, but that’s truly asking for unnecessary trouble.
As for having someone else represent you, it’s probably possible, and definitely something to run by an attorney because the government offices are very officious about getting such paperwork right, and it usually requires the involvement of the peculiarly Spanish institution of the Notario who is expensive and (to you) worthless.
Once you have submitted the application (which can be done by mail - not even one trip to the police required), you will make two later trips to the police (assuming all goes properly) - one once your new permit has been approved, to move the process along, and a second one a few weeks later to pick up the new plastic.
(Yes, EU folks, we non-communitarians do still get real plastic).
You also noted that this applies to you and to your family members. Do all of your cards have the same expiration date? If not, are they all within 45 days of each other? It will be far more convenient for you if they all are within 45 days of each other, so that no earlier than 45 days before the latest expiration, and no later than a few days before the earliest expiration, you can submit all of the paperwork together. Otherwise, plan for duplicate or triplicate bureaucratic time expenditures.
You asked another question, how long can you be away? The rules, when I got my first permit coming up on three years ago, were that at no time during my entire first five years of residency could I accumulate more than five months away from Spain except for absences caused by the needs of your Spanish job, or humanitarian purposes. I advise you to check with an attorney if you will approach that five months accumulated. I believe that there was also a limit of 90 days at any one stretch away from Spain.
Further, while you’re “legal but without normal papers”, there is a limit of 90 days away (again, see exceptions and caveat above) AND for any exit and re-entry in to the country during the “legal but without normal papers” period, you must get, in advance, a permission to return. It’s a ?10 per person per instance two hour waste of time standing in a line. Not hard, and there’s no question you’ll get it each time, but you do have to do it.
Watch out: You’ll be asked for how long you want the permission to return to be valid. You have to give a start date and a run time. *The whole time you ask for on each instance, not just the time you are actually away from Spain, counts against the 90 days accumulated maximum time away*. So give them the date of your actual departure (or one or two days earlier if you think you might flex), and a period of time away which is very little longer than the actual time you will be away (again, plus a little if you think your trip will stretch).
Stupid bureaucracy, but if you know the rules and follow them, not particularly difficult.
Huge caveat: That was eighteen months ago for me, and the rules here do change often. Check with an attorney to be sure, and hopefully someone with more recent experience will also post.
Disfrute!
-Jay
Barcelona