You can use this process after you've already designed an encounter to determine its difficulty or you can use it to start an Encounter from scratch with a target difficulty in mind.
Start by making a note of the Exp values that define the four difficulty categories for your group. For each adventurer in the party refer to the Encounter Difficulty Exp per Character table, noting the exp values for each character in each category. Then for each category add the exp values for each character in that category to determine the difficulty thresholds for encounters.
Next, select each creature you want to include in your encounter. Add up their Exp Values, which can be found in their stat block next to their Challenge Rating, to get the encounter's Exp Value.
Adjust the encounter Exp Value by the multiplier found in the Encounter Exp Multipliers table according to the size of the adventuring party.
Compare the adjusted encounter Exp Value with the difficulty thresholds you calculated in Step 1 to get an estimate of the encounter's difficulty. Adjust the encounter accordingly.