Thank you, it helped me a litle bit. For the next guy who will read this, the best answear I've found is this one :
"Where do the connections that show a ground symbol go to?
Generally speaking, take all the ground points, connect them together, then connect them to the ground lug of the input or output jack. Take the power supply (i.e. battery) and connect it to the ground lug as well. If you want the input jack to switch your battery on and off, then connect the lug to ground as before, but connect the battery terminal to the middle connector (i.e. "ring")of a stereo jack. When a mono plug is inserted, the ground circuit will be complete and the switch will be on. Note that some circuits connect the + or positive terminal of the battery to ground (positive ground circuit), although it is more common to connect the negative terminal to ground. The sleeves of the input and output jacks usually go to ground as they are usually connected to the metal casing and the ground lug - unless they are isolated jacks (in which case you need to ground both jack sleeves with a ground wire). In general, metal jacks are not isolated from metal enclosures. If your box is painted you might consider scraping off some paint so the jacks can make contact with the metal on the box so the box can act as a shield; lock washers may do this as well. "