To determine how many experience points a Pokemon earns after defeating a Pokemon, you just divide the level of the defeated Pokemon by two. For example, if you defeated a Level 5 Pokemon, you would divide that by 2. This equals 2.5 Experience points.
If you are high enough Level to earn a Trainer Bonus (Lv. 1 Trainer or Lv. 2 Coordinator), you get more Experience points. To determine how much a Pokemon with a Trainer Bonus earns, you multiply by 0.75 instead of 0.5, as you would for Pokemon without Trainer Bonuses. So if you defeated a level 5 Pokemon, you would get 3.75 experience, which rounds up to 4.
If you are a Level 5 Trainer or a Level 6 Coordinator, you get the Next Level Trainer Bonus, which is full experience. This means that if you defeat a level 5 Pokemon, you get 5 experience.
Experience points may only increase by increments as small as halves, which means that if an EXP gain contains an increment smaller than that, you'd round it up to the next half. For example, if you defeated a level 5 Pokemon and you got the Trainer bonus, you would round the 3.75 EXP you got up to 4.
