You measure around her neck - you want a measurement that is comfortably firm (if you've ever been measured for clothes - same sort of thing) and that won't slip over her head. Then the same for just behind her "elbows". Maybe the distance between where you want the neck-bit and the chest bit as well, if she's a patient girl and hasn't run off with your tape-measure yet.

"Close enough" is good enough, the neck- and chest-straps can easily be made to expand or contract to fit. Then you pick the pattern you want from what is available for the collars. And Robert is your mother's brother.

Just about anything light-weight will do for a lead. Dog-lead, shoelace, extending-cord lead, Rover's tried them all.
