Flight dispatch is generally done using software by Jeppesen - it costs millions and does all the fuel calculations etc. If you wanted to do it yourself, you'd need to start wading through tables in the Boeing performance manuals and it would take you roughly until the end of time to get it anywhere near accurate.
Once the computer generated flight plans are received by the flight crew, we then check the weather at departure, destination and alternates. If they are fine, there is normally no need to adjust the calculated fuel. If the weather is bad, an airfield is closed or other delays are anticipated, the fuel can be manually adjusted.
An alternate airport is chosen because it's near to the destination and has enough facilities to cope with your aircraft.
Regarding derates, the Boeing uses an assumed temperature method - you have to wade through more pages of tables to work this out - they are airfield specific.