Some left handed electric guitars will just NOT adjust correctly with the standard methods available like the bridge saddles, nut filing adjustments and the capability to set the bow of the neck itself.  Adding shims to the bolt on neck could be compulsory to get the guitar to play correctly at all locations up and down the neck.  

A nut shim acts as a spacer between the nut and neck raising and lowering the space between all the strings at the bottom of the guitar, beginning at fret one and moving towards the body of the guitar.  Attempt to imagine the strings of your left handed guitar as a flat plane and the fret board as a parallel plane running beneath them. 

The angle of the top plane which has the strings is controlled by the position of the tremolo and the nut.  The angle of the lower plane which is the fret board is controlled by the neck pocket of the body.  If your setup is excellent these two planes will have an equal opening between them at any point. 

If your guitar doesn’t look this way try changing the peak of the tremolo itself first.  This can sometimes look after the issue unless you find your action becoming too high or to low likewise across the length of the fret board. 

If fixing the top of the tremolo corrects the issue but leaves you with too high or low of an action ( opening between the strings and fret board ) then you must turn to employing a shim.  You could need to block the tremolo before you start the shimming operation. 

Shims can be discovered and used in 2 different areas of the neck.  One is under the nut and the second is without delay under the heel in the neck pocket of the body.  Nut shims are typically made out of a number of thin sheets of brass.  In all cases you can produce your own shim if needed by employing a sheet of heavy stock paper like a card. 

For shims in the neck pocket you may need to double the paper stock 3-4 times to get the essential thickness required, then trim to fit correctly.  First establish if the space between the strings and fret board is to shut right up against the nut itself.  

If this is the case then add a shim under the nut and raise the tremolo by fixing the tremolo posts until the strings are an equal distance from the fret board down the entire length of the neck.  If your strings are higher on the Low E side, place the shim in the neck pocket under the heel of the neck and parallel with the length of the neck on the Low E side, which raises the entire neck down the Low E side when it is bolted on. 

Before doing this check the angle of the tremolo to be sure it is flat against the body and not floating at an angle.  Again before doing this check the angle of the tremolo to guarantee it is flat against the body and not floating at an angle.  If your strings become higher the closer they are to the body place the shim across the neck pocket in the final analysis nearest to the neck pickup.  

If your strings become lower across the higher end frets place the shim across the end of the neck pocket farthest away from the neck pickup.  This may raise the headstock away from the back of the body, raising the strings up from the end of the fret board, nearest to the body.  In some examples you’ll realize that you need the shim to raise just one corner of the two planes as shown above.  In these rare cases, make a smaller shim and place it in the satisfactory area of the neck pocket. 

Naturally on removing the nut from the neck or the neck from the body, if you find a shim already there work out what action it was doing in the initial place then make the mandatory corrections.  Shimming a guitar neck is a skill.  If you look at it scientifically, as angles against an aircraft, you can understand the how and why of adjusting a guitar neck this way.