Binders will have a "cut width" and a "finished width" for the binding. Cut your binding strips exactly the width of the cut width. If they are too narrow or too wide, they won't feed into the binder properly and your binding may look messy and uneven. Now, having said that, there *are* times I purposely cut the binding strip a bit wider than required. This is mostly with very thin knits with a lot of stretch. The binder tension will stretch the strip too much making it feed through narrower than a less stretchy strip. The little bit of extra width counteracts that.
If using a knit, cut the strip across the stretch of the fabric so that the stretch runs the long length of the strip. If using a woven, cut the strip on the true bias (45°).
After cutting my binding strip, I cut a point at one end to make it easier to feed the strip into the binder.
I use the point of a straight pin to "push" the binding through the folding area.
After the end has poked out of the binder, I grab the strip with my fingers and gently pull, allowing the strip to be properly folded by the binder.
I keep pulling until I have a long enough length to pull around toward and under the foot.
After the binding strip is positioned under the foot, feed the other end through the serpentine guides (if your binder has them), as shown below.