It can't.  To do it right you have to take everything apart, and throw away
the
dryer or accumulator (depending on which one you have) and throw away
the orifice tube or expansion valve, depending on what you have, and replace
them.

replace ALL o-rings with new ones, and make sure they are the o rings
that won't disintegrate in the presence of PAG oil.  (get them from an AC
shop)

If this was an R12 system, it won't have barrier hose.  You can use the
existing hose if your a cheapskate (I was) but it will leak at a very low
level  (ie: you will have to refill the system probably once a year)  This
trick only works because the existing hose already has a little bit
of oil driven deep into the rubber and that helps to seal the hose - brand
new non-barrier hose will leak like crazy if you use it in a R134a
system.

Also if this was an R12 system it will not be as cool.  Where you will
really notice this is when airflow through the condensor unit is low,
such as at idle.  You can replace the condensor with an aftermarket one
that is more efficient, like a parallel flow unit, which will restore
cooling.

Fush each tube including the flex lines with at least a quart of mineral
spirits
and run at least a minute of compressed air through to dry out the mineral
spirits.
There is a flush gun that makes this process much easier, that you can buy.
Make absolutely sure that the compressed air your running through
this is dry, not oiled.  (many people run oilers on their compressed
air lines to keep the air tools lubed)  Also make sure it's got a water
trap.

Return the refrigerant to the store.  The pre-mixed stuff is rediculous.

Since this is a new system and your R134a you want to use PAG oil.
Ester oil is for retrofits where they are not replacing the compressor or
taking anything apart, and crossing their fingers and hoping the works don't
blow over the  mountain.

There are 3 different PAG oils   If your system came from the factory with
R134 in it, there will be a sticker somewhere specifing which grade of PAG
to use.  If yours is previously a R12 system, then it probably doesen't
matter
much which PAG oil you use.

When you do this, once you have the system completely bone dry, and all
the hoses back together, before you put the compressor in, pour a little oil
into it's ports and turn the compressor to distribute the oil inside, so the
startup isn't dry.  Once the compressor is fitted, if you have a system like
a
GM system where it's got an expansion valve, before you put the valve in
you can use a funnel and pour the oil down in there.  (since the pipe
points up)  the oil mostly hangs out in the accumulator or dryer, so
you can also pour it in there.

Pull a vacuum on the system for at least 2-3 hours.  Overnight if you
can, is even better.  This will remove residual water as well as boil the
water out of the PAG oil  (which will suck it up wherever it can get it)

Ted