When you take your shot, look carefully for the exact spot where the deer was standing when hit. Mark a boulder, or a large oak tree, or a stump that is nearby that spot.
Also remember which direction the deer ran.
After a few minutes go to the spot where the deer was standing. You may see a big splash of blood. You may see some white hair. You will be able to see deep hoof marks, where the deer sprung away as he was hit.
Mark this spot with a flag. I take a roll of toilet paper into the woods with me. Put a couple pieces on a bush 5 feet high. If you don't mark this spot now, you may be unable to find it again.
From this point there is a trail that leads to your deer, you just have to know how to read it.
Hopefully there is a good blood trail, splotches of blood a couple inches wide.
However, you may have little drops of blood, only as big as a bb. If so, you have to get down on your hands and knees to see them.
I have trailed a deer for 100 yards on my hands and knees, in the dark, following blood drops the size of a bb.
Be sure to carry a flashlight, as the minutes just before dark are some of the best deer hunting. Those little blood drops light up under a flashlight like they have little light bulbs inside.
Every 15 feet or so put up another flag.
Some deer go in a straight line until the drop, maybe 100 yards away.
Many times, however, a deer will go about 30 yards, and make a sharp turn, and from that point go straight until they pile up. This turn may be as sharp as 180 degrees.
Wounded deer almost always go downhill.
The reason for the flags is that, you may go 50 yards, and the blood trail may conk out. Then, you can look back and see the exact path that the deer has taken.
A good trick at that point is to just walk in that same direction, even if you can't find more blood if the deer is well hit you will find him in the next 70 yards.
I have gotten the best blood trails on a lung shot with the .50 roundball from a muzzleloader. It looked like a 7 year old boy was walking through the woods with an opened, full gallon of red paint in each hand, every 4 feet on either side a big blotch of blood about 5 inches across.