I'm trying to create a bent grid in Inkscape using interpolate like in this tutorial. However, when I follow this instruction word for word, my interpolation looks like this:
How do I make sure it interpolates as it does in the tutorial?
Your start and end curves have different directions. You must draw them both for example from up to down.
If you have a trajectory, its direction also affects the result, but this problem is caused by the different directions of the start and end curves. If they happen already be drawn otherwise perfectly, you can remove the effect, break apart the combination path and reverse one of the paths (Path > Reverse)
Here's an example what is caused by the different directions. Someone can also want this. It's fixed in the right image.
BTW. The horizontal curve in the middle is the used trajectory.
You can now edit the trajectory path using the Edit Paths by Nodes tool