I've found that when stalking IF you keep a hedge or a large tree obscuring you from their view, you can get closer than merely stepping slowly. You have to slowly lean around the blocking tree or hedge to peer around it to keep track of the squirrel, but if they are busy digging out an acorn, they will stop when they hear you step, and then continue if they hear nothing else and don't SEE you moving.
I have better luck entering the woods where squirrels may be foraging, by entering from a very thickly overgrown area, where it's tough for me to see them from a distance and tough for them to see me. Once I've negotiated the brush enough to see into a wooded area, I sit still and observe. I've found that while it's great to see a squirrel or two scampering in the trees as I walk into the woods from the fields, they are usually scampering to get into a nest because they've seen me coming.
If you are in an area of oak trees BUT you don't see the large leaf nests marking squirrel territory, look hard for openings in the tree trunks. That black spot, 6 feet or more off the ground, where a branch has broken off at the trunk..., MAY be a hole, AND look at the bark near and around that spot. I've noticed that the squirrels, like bats, sometimes discolor the bark beneath a hole from constantly climbing in and out of it.
LD