The service valve near the dryer (with the site glass) is the High side
port. You may also recognize the low side  by the fact that the refrigerant
line is a larger diameter than the high side line. When you purchase the
retrofit kit for your vehicle it should come with instructions that will
help in getting the job done.  You may want to look for the refrigerant
decal affixed somewhere under the hood of your vehicle. It will have the
proper R12 capacity for your vehicle on it. When you recharge the system it
is recommended that you only put in 80% of r134a that was recommended as the
total capacity for R12. Just be sure to read and follow the directions on
the retro fit kit carefully. You would need considerable experience to
determine the correct refrigerant charge by using the pressure gage
readings. The are some generalities that apply, but the correct pressures
readings may vary depending on many different conditions.

BTW, it has been my experience that the 86 Civic did not have a large enough
capacity condenser to handle retrofit to r134a very well. It will work and
cool somewhat, especially on the highway, but around town in stop and go
traffic it will not keep up with much of a heat load.

You may want to consider using an after market R12 replacement such as
Freeze 12. It has operating pressures closer to that of R12, and generally
offers better cooling capacity when used as a replacement in that sort of
system. Keep in mind that retrofitting with Freeze 12 will limit the number
of repair shops that will consider working on your system in the future
though. Many are not equipped to handle reclaiming and recycling of
refrigerants other than R12 and r134a.

No, you don't.  There's a glut of R12 on the market these days, what you
want to do is to take the EPA test to get certified then go buy the
chargette
cans off Ebay like you used to be able to do from the auto parts store.

The only way a retrofit works for the long term is to completely purge the
system using mineral spirits, replace the compressor & accumulator, then
put new PAG oil in along with refrigerant.  And like the other poster said,
your cooling will go to sh.t since R134a does not cool as well and needs
larger condensors.  Oh and I forgot, you have to replace all your hoses with
barrier hose as well or the R134a will just leak out through the hoses.

Ted