Every instrument-rated pilot knows the 1-2-3 rule: One hour before or after your ETA, you need a 2,000' ceiling and three statute miles of visibility. Otherwise, an alternate is required.
Your alternate has to either be VMC or have an instrument approach if it's IMC. If the alternate has a precision approach, you need a 600' ceiling and two statute miles of visibility; for non-precision, it's an 800' ceiling and two statute miles of visibility.
Non-standard alternate airport minimums may exist. You'll find them in the TERPS, under IFR Alternate Airport Minimums, and marked with an "A" on the approach plate. Your alternate airport also must have an approach that does not require GPS.
As always, remember that 91.203 requires you to know everything about your flight, including the weather. Conducting a RAIM check would be included in this preparation, both for the departure, primary arrival, and alternate airport. It's a given that a current GPS database be running in your GPS as well.
If you use an iPad (or any other tablet), you should carry paper charts, approach plates, or another iPad you have fully loaded, charged, and tested for any glitches beforehand.
OK, so let's say you get everything lined up, dialed in, and you take off. The procedure for going to your pre-filed alternate is to advise ATC that, for whatever reason, you must go there, be it an inflight weather update or having to go missed at your primary airport.
They vector you, and if it's a towered airport, the Tower closes your IFR flight plan for you. If it's non-towered, you can cancel with ATC, call a number they usually give you, or call FSS.
Failure to cancel an IFR flight plan keeps the airport restricted from any other IFR operations for a while. Suffice it to say the FAA will not be happy with you for failing to cancel.