FLOWERING PLUM TREE

Depends on the soil, If in good heart then all you need is some garden compost added to the soil at planting time, Dust LIGHtLY with bone meal, not huge great handfuls. Half a cup to the square metre is plenty
Depth is easy, plant at the same depth as it was in the pot. Do not plat deeper of you get stem rot. Water well before and after planting and then unless you get major drought leave it to go looking for water in the soil. If you do have to water cos of drought then Drown it. 1 gallon is useless 20 would be better.(but only if really really needed.)
Cannot think of anything else, other than a good strong stake to fasten it too until the roots establish.

Agree with Owdboggy that 20 gallons is better than 1 - this is so that the roots go DOWN into the soil - if you just give it a little water and often, then the roots will tend to form too near the surface and then it will suffer if you then don't water it. However, for a new tree, personnally I would recommend watering often in its first year to get it established - to get the water to go right down and persuade the roots to go down rather than up, you could put a peice of plastic tube in the hole when you plant it with the end stuck up above the ground and water it down that. Also, are you getting a bare-root tree or one in a pot? If bareroot, you need to get a move on - it's really a bit too late for planting bare root trees now as they need to be fully dormant before planting. A pot grown one should be fine tho. Don't allow anything (including grass) to grow within about 2 foot of hte base of hte tree as they will compete with the tree for nutrients and water. Soil preparation is the key!

THANK YOU SID AND OWDBOGGY!
I AM GETTING ONE IN A POT. THE ONE THING WE HAVE PLENTY OF IN THIS STATE IS RAIN. DO I ADD FERTILIZER?
WHAT ABOUT PESTICIDES?

As I said only add fertilser if your soil is poor. If you have lots of strong growing weeds then your soil is good If the weeds struggle then add fertilser, better well rotted manure or garden compst than chemicals though. Pestcides are definitely not needed.
As to watering in the first year, our old gardener (sounds good, but we lived in a hotel with extensive grounds and the company paid for a chap to maintain them), always said that you watererd a tree once and that was when you planted it. "Soak it in an leave it be" He had planted hundreds of trees in his lifetime (most in Royal gardenns as he had worked for Them!).
The more humus (dead plant material) you add when you plant the better for the tree. In Britain there is a product specifically sold for adding to soil when planting trees, not sure of the name though.
http://www.rhs.org.uk/advice/profiles0903/soil_level.asp

Dig some into the base of the hole and mix with the soil there then, mix the compost with the soil you dig out of the hole and put it back in.