These tutorials by Thiago should go a long way to helping you;
Editing Paths on a Spline
https://docs.google.com/presentation/d/14tKynRTGvvdWrVY4v0V5aEOkzsh7MFxcib41t5bvlz4/edit#slide=id.i0
Creating Splines
https://dl.dropboxusercontent.com/u/17534772/OMSI/Tutorials/%233/Creating splines.pdf
Hope these help!
However going through these tutorials they are helpful but don't really cover what i am after. Im after how do you add a human path on a empty bit of pavement?
Ok i think that i will just use a invis human
Do be aware though that it has weird values, it needs to be set 2+ metres high to be on the ground.
If you plan on using the invisible sidewalk spline I highly advise that you make a copy of it in your own unique folder. Maps keep on overwriting the spline with "2 metre too high i fix".