Well can anyone one help me with this,also i want this so only the arm is involved mostly. e.g not like bodybreaker.

Depending on the mod.
Usually in aikido servers people would place thier arm an the side of the head, and contract all thier arm joints. This would act like cutter and pop the head out.
in other servers that have a less dm threshold usually a solid smack with your arm would do the trick.

There are no easy or hard ways to decap. Just practice and maybe watch somebody else's replays with shift+p to see how they do it or get some inspiration.

Aim for the neck! If exposed, and if you hit hard enough, hopefully it'll be a decap.

Line your tricep up with his neck joint and rapidly contract everything on that arm.

decap is almost always lucky.
Its same like question 'how to hack hampa's acc'.
There are a lot of possibilities. You have to find them yourself.

There's no easy way to decap without a given scenario.
If you're asking for a move that'll decap someone everytime for any move, you won't find it.
Practice and find out yourself.

Here are some easy decaps.

You have try to hit right at the joints, that's easiest way, and if you hit in mid of torso you could take more parts at ounce like this replay

Hit the neck joint hard. If it doesn't decap, hit the neck joint harder, or at a different angle.

All of these other suggestions people are making are pointless and based on a matter of opinion.