I need to mirror the half part of the rigging of my character, I remember it was something with the scale or rotation.. does anyone know which was the right solution step-to-step?
In order to mirror a rig down the very middle, you'll need the 3d cursor reset (ShiftC) and used as your 'Pivot Point' (.). That'll determine the mirror center, so long as the rig itself is placed correctly in the center (0,0,0). Now Select any half of the armature (excluding the spine/middle) and duplicate it with ShiftD, next press CtrlM + (the axis you are working along, usually X) to mirror it over to the other side. With the selection still instact, press W > Flip Names to update the names so something like upper-arm.L becomes upper-arm.R.
After doing this, the mirrored bones might have a different roll from the other side. To fix, select the original bones (can just invert selection with CtrlI if you still have the mirrored side selected), enable X-Axis mirror under Armature Options in the toolshelf and press CtrlR to fix the roll.
I just tried to name the bones correctly on one side (ending with .L in my case) then while the pivot center was at (0,0,0) as recommended here I pressed W and choose Symmetrize. BEST THING EVER! It automatically makes a duplicate with the right names, not adding the .001 at the end making them more complicated and best of all the X axis mirror option works! This means I can even copy the keyframes of one bone and paste mirror at the other if the pose is symmetrical!
It helps to have X-ray and Names enabled in the viewport so you see them all.
Then, having a left and light side ready, in edit mode you can activate the X-Axis mirror and it works. In Pose mode you can copy the bone properties (rotation and position) with Ctrl+C
I would recommend to use the x-mirror(makes work easier down the road). See the duplicate that NoviceInDisguise linked.
Manually, you can do this: Set 3D-cursor to center. Then select rotation around 3D cursor. Choose the bones you want to mirror, duplicate them(shift + d), press S for scale, then x and then press -1. Duplicate bones will mirror over z-axis.
Here are your steps :
I just remembered, to mirror a half part of the bones/skeleton you have to take all the bones on the half (left or right) part of the mirror, then press S (Scale) and -1, then rotate it with R (Rotation) on 180°, plus pressing X, then you will have the half part like a mirror. Also, the pivot should stay at median point pivot.