Basically, sewing is solid geometry, and preparing to sew is the process of
translating plane geometry (the two-dimensional pattern) into solid geometry
(the three-dimensional garment).  Intellectually, garment construction is
probably closer to those IQ tests that show you pictures of a series of
three-dimensional shapes, and then from another set have you pick out the one
that best fits in the sequence, than it is to flat needlework like embroidery
or needlepoint.

Generally, when I am going to use a pattern that I haven't used before, the
first thing that I do (WAY before I put it on the fabric and cut anything out)
is sit down and read the instructions all the way through.  I use any handy
scrap of fabric to do the folding as I go.  If at any point, the fold doesn't
come out in a logical way, I stick a post-it next to that particular
instruction to remind me that I'm going to have to do some thinking.  If it
really, really doesn't come out right, I may even modify the pattern before I
cut.

I never hesitate to modify the instructions when they either 

(1) present an unnecessarily complicated way of doing something for which I
know a simpler technique (I always apply collars the same way, no matter what
the instructions say); or 

(2) just don't produce the desired result (remember the long-past extensive
discussion of how to get a short-sleeve cuff folded right on one of the 1940's
dresses from the patterns in the "Time Machine" book).  

Otherwise, I do follow them, on the presumption that the pattern designer had
a reason for doing it that way.  For instance, several people on the list had
trouble with the collar instructions for the PC Molly's pajamas pattern.
However, I did them step-by-step and they did work if one followed the old
technique (not used since the 1950's, I think) of clipping the seam allowance
right to the seam line on the crucial corners and then reinforcing the seam
after the clip was done).  I've been really glad, since then, that I took the
trouble, because Molly's pajama top can be easily modified into classic open-
neck collar/and/lapel sports shirts for the boy dolls, and also works to give
Molly herself the v-neck/collar/lapels blouses that we wore in the 1950's
(Ship & Shore?--I think that was the universal brand name).  Just reverse the
direction of the buttons, and Molly's pajamas also make excellent boy doll
pajamas.

    Source: geocities.com/mom4max