I M M S ! !
instantiate & mirror & merge & subdividing
from now on there will be four words
original , inst , merged , subdiv
it is objects' name ! don't be misunderstood
2. detail modeling
1. instantiate & mirror
. freeze all transformation
. instantiate original
. scale inst by x = -1
2. merge & subdivision
. merge original and inst like below

then you will have polymesh. rename it by merged
subdivide merged like below (this is what roger rosa call mode 1)

. hide inst and merged then what you see is original and subdiv
at this time you will check what you were doing in explorer and schematic view
notice the relation among four objects in schematic view

but they are overlapped in the viewport
it is hard to tag and move points to adjust original
therefore original needs to be viewed as wireframe
what you need to do is two things
1. select original
model > property > display

2. untoggle override object properties

3. layer
what we are going to do is adjusting the point in original in tag mode ,
so inner object (subdiv) doesn'have to be selected

. select the subdiv
, create new layer ( layer pannel > new layer )

. name the new layer by sbd
you will check this in explorer (press L)

. press 6 to open layer control pannel
toggle or untoggle like below , now you can preview by dragging Q interactively

Ok . well done

when your are satisfied
. delete inst
then merged and subdiv will be deleted also ( cause it is related )
what you need is reasonalbly subdivided original
1. instantiate and mirror original ( don't forget to freeze before instantiate original )
2. merge the two and rename it as final merge
spread the final merge in explore and open the geometry approximaton property
(this is what roger rosa call mode 2)


polygonmesh tab > tweak mesh subdivision level (1 or 2 )
At last it is finished !
However, why didn't we use method of mode 1 instead of mode 2
it is because texture propagation
In mode 2 method if original is textured , automatically texture is propagated to subdivided objects ,which in mode 1 is impossible
But in mode 2 it has two kinds of subdvision rulei, that is catmull clark and doo sabin,
and objects can be selectable and adjustable which in mode 2 impossible
for more information roger rosa
Anyway
after adding texture
and basic 3 lighits