Howdy from Texas,

Something that I don't think anyone has mentioned is that you will do
your best work if you are illustrating something you love. Someone
who knows and loves roses and orchids might do a technically competent
job illustrating grasses, but the work won't have the "involvement" or
"investment" of someone who has a gut relationship with the group.
That said, drawing is also a way of understanding--you notice a lot
when you have to draw something, so you may find yourself falling for
something you draw.

Everyone was right when they said the features to be illustrated will
vary from plant to plant or group to group. For
example--Nepenthes--tropical pitcher plants. For these guys, it's
going to be the leaves and pitchers--shape, size, marking, etc. The
flowers are extremely secondary. For orchids, it's mostly about
flowers and their details. For things in the carrot and mustard
family, it's fruits and leaves. For a tree, it might be fruit,
leaves, and bark. Some families have specialized structures that are
like fingerprints for each species. You'd put the other bits in, of
course, but the emphasis will change from plant to plant.

M. Reed