First, you need to know the value for the bond dissociations of C-C bonds, C-H bonds, O-H bonds and C-O bonds. Let's start with the combustion of methanol. As always, an equation is needed.
2 moles of methanol + 3 moles of oxygen gas = 2 moles of carbon dioxide + 4 moles of water
: The bonds found are:
2 x C-O bond
6 x C-H bond
2 x O-H bond
3 x O=O bonds
4 x C=O
8 x O-H
These will all have their particular values for bond dissociation, in other words, how much energy it takes to overcome these bonds and break them, which will come in the form of "x kilojoules per mole."
Then what you have to do is add all the bond dissociations up for the reactants and products. The bond dissociations of the products are always written as negative as it has taken energy to make them, as opposed to energy given out breaking bonds. To calculate the bond enthalpy (total energy of the reaction), you subtract the reactants' bond dissociation values from the products, and you're there.