Preflight Preparation

Everyone has to find you own routine in doing preflight preparation. Here is how I usually do it…

Days Before your planned flight

Checking general weather outlook at http://www.wunderground.com. Search for Guatemala and places along your planned route. If going from Guatemala to Rio Dulce, I would check Guatemala, Zacapa and Puerto Barrios. Click on the Hourly Forecast, then select your planned day of flying. Check the time of you flight and have a look at wind, wind direction, general weather and cloud cover forecast.

Day Before your flight

Check the weather again at http://www.wunderground.com.
Get the TAF for your airports and stations along the route. http://aviationweather.gov/adds/metars/. Type MG into the text field and select TAF in the below checkbox. This will display all available TAFs for Guatemala.
If you have a long route planned, check the winds aloft. http://aviationweather.gov/iffdp/. Click the Area A and observe temperatures and wind aloft.
Check for NOTAMs under https://pilotweb.nas.faa.gov/PilotWeb/.
Calculate Weights & Balances.
Calculate route, heading, speeds, fuel consumption, etc.

Same Day

Essentially I would repeat the same as above. When checking for TAFs, I would pay special attention to the METARs and use the historic feature of the FAA to display the last 36 hours of METARs. It sometimes reveals a tendency.
Check the weather again at http://www.wunderground.com.
Get the TAF for your airports and stations along the route. http://aviationweather.gov/adds/metars/. Type MG into the text field and select METAR and the history function.
If you have a long route planned, check the winds aloft. http://aviationweather.gov/iffdp/. Click the Area A and observe temperatures and wind aloft.
Check for NOTAMs under https://pilotweb.nas.faa.gov/PilotWeb/.