This cardigan is made with knitting needles, using the reverse rib stitch.
Start from the back part, make the reverse rib stitch to start making increases on the sleeves. Make the hole for the neck, and continue making the front part adding some stitches on the front, making the decreases of the sleeves, and finishing the front part. Take the stitches of the other front and repeat the same.
At the end, you only have to sew the sides and the bottom part of the sleeves.

Here you can see the free video tutorial https://lanasyovillos.com/en/clothes/baby-mustard-cardigan/