Order doesn't matter as long as they're alternating. That means in the first step, for example, you can do
L M L
M X M
L M L
or
M L M
L X L
M L M
(where L = Linseed, M = Mandrake, X = Scarecrow position if you're using the large garden bed)
Just make sure you plant them in order instead of planting all of one seed first, then planting the other. As for soil, either Grade 2 or Grade 3 Thanalan Soil for all.
