On this page of the Assassin's Creed Valhalla walkthrough guide, you will find a detailed walkthrough description of the A Feline's Footfall mission.
Master blacksmith Ivaldi needs A cat's footsteps to forge the magic cord, Gleipnir.
Collect a cat's footfall
You can find a clue regarding where to find this ingredient by reading a note in your inventory' story items section.
Go to Asgard and go near the stairs shown in the image above. From here you must climb to the very top of the massive arch.
Passing between the pillars, you will find one of Freyja's golden cat statues. As you get closer, the cat will melt away into the air and move slightly forward.
Now you just have to follow the cat's footsteps, jumping between obstacles. As you're sliding down the rope, you will have to let go at just the right moment to land on a narrow pillar.
When the cat leads you to a vantage point, jump down and land in the haystack to get the ingredient you need.