How to fix cracks in lath and plaster?
Spackling's best use is to fill holes made by pictures, curtain holders, etc.  To fix cracks in plaster, use Plaster of Paris or Durabond, not spackling compound.

Cracks are caused by moving or shifting, so one needs to make sure the existing plaster and corresponding substrate (in this case the lath) are solid.

A simple test is on both sides of the crack in several locations gently push the plaster.  If there is movement, additional repairs may be required beyond a simple crack fix.  For example, one may need to replace the lath or substrate.  A simple crack may also turn into a large area where the plaster needs to be re-attached to the substrate.  This is the case where the crack is hiding a larger issue of crumbling or decaying plaster and will need a larger scale fix.

I would advise doing these tests on all your cracks to determine the whether the plaster is just cracked or is suffering from bigger and more labor intensive issues. 

If the plaster and substrate is solid, then to fix the crack do the following.


Remove loose or excess plaster in crack using a utility knife
Enlarge the crack by creating a V notch with the bottom of the V being the line of crack
Clean or vacuum any loose debris out of the crack.  Wipe any dust with a damp rag to prepare the surface for the new Plaster of Paris or Durabond. 
In your case, I would not use Plaster, but instead use Durabond.  I would go for a first coat of non sandable Durabond.  Plaster is hard to work with so using Durabond will be easier.  The nonsandable Durabond is stronger than the sandable durabound.  Follow instructions on how to mix the Durabond.  
Use a joint compound knife and work the mixture into the crack, removing any excess.  Do not apply extra as it is non sandable.
One the first coat is dry (it should also shrink a bit), switch over to the sandable Durabond.  Apply 1 to 3 coats (it will shrink a little each time) and sand in between.  The final coat should be very light and then sanded, primed and painted.


Plaster is hard to work work with and requires a skilled laborer to apply.  So anything beyond a simple crack fix, I would go to a professional.