So I got a question about blue soul signs, I know how they work I have already summoned a phantom to my world but how to I put one down myself ? I got killed by the tower knight so I am in soul form but I still cant use the blue crystal in my inventory.
anyone knows what the problem is ?

You need to be in soul form to place the signs, and you need to have your body to be able to summon other people. Also any objectives you complete when you are summoned won't be completed for you (only for the guy that summoned you) 
 
Also (this is probably the problem for you) there are areas where you can't summon (generally the area that bosses are in) so go to an area before/after the bosses to be able to summon.

Yeah you can't use it in that room, you have to go just outside the room :) 
 
And yes you will get your body back! But that also opens you up for invasion ;) I tend to do most stuff (including bosses) in soul form to avoid invaders myself ;D

If you want an easy way to tell where you can use the blue eye stone, equip it into one of your usable item slots and just walk around till it lights up.