Pendulum Summoning is rather easy, if a little difficult to understand at first.
So the first thing you need to know is that Pendulum Monsters can be treated as either a Monster Card while in the Monster Zone, Hand, Deck or Extra Deck or a Spell Card while in the Pendulum Zone. In the description they will have 2 effects. The first effect or top effect is the effect that monster will have when its treated as a spell card in the Pendulum Zone. The Second effect or bottom effect is the effect it will have as a monster.
You may also have noticed two little arrows(one red the other blue) on this cards with numbers beside them. These are known as Pendulum Scales. Each Pendulum monsters scales will vary but both scales will be the same.
In order to Pendulum Summon you need to have 2 pendulum monsters in the pendulum zones (one in each zone). Simply state to your opponent that you will Pendulum Summon (since it's polite and let's them know what you are doing) and Pendulum Summon any number of monsters from your hand or face-up from your extra deck to your side of the field. The levels of the monsters summoned must be lower than the highest scale of your Pendulum Scale and Higher the the lowest scale of your Pendulum scale.
For example: if your Pendulum scale is 1-8 then you can only Pendulum Summon monsters whose levels fall between 2-7. You cannot Pendulum Summon any monster whose level is either the same as either scale or falls outside your scale.
This is considered one summon and all monsters summoned are summoned simultaneously.
Now, upon destruction Pendulum Monsters are not sent to the graveyard. They are instead sent to the extra deck and placed face-up.
Pendulum Monsters in the Monster Zone are susceptible to any effect that affects monsters (as they are treated as monsters). And Pendulum Monsters in the Pendulum Zone are susceptible to any effect that affects Spell cards(as they are treated as Spell Cards).
The Number of times you may place a monster in the Pendulum Zone is up to twice per turn.
I hope this helped.