If somebody’s paying for this, get a wide bluescreen, not small. You want her to walk over the longest stretch possible so you have as many walk cycles as possible to choose from.
Mark the floor in regular intervals so her pace is the same length throughout.
Get a metronome so she walks at the same pace.
Shoot from as far away as you can to minimize perspective distortion.
Since she’s in silhouette, dress her in colours opposite the background. Maybe black, then light the background well. Yes, the background should still be a saturated green or blue.
This will be a lot of work matching her walk. Do some tests, maybe on a wall with a bright sky behind. Hey, why not do the final that way? Anyway, do the tests to know what needs to be in place on set, how to shoot it, and so on. Then try to cur a cycle together, with sitting and so on.