Check your top tension. It might be too tight if the fabric is bunching. The stitching should meet in the middle of the fabric but not be so tight it pulls. Loosen the tension a tiny bit and sew on a scrap, then loosen again and test it out. If the thread loops on top or bottom then turn it back. Once you get the tension right it should sew better.

Sewing with a slippery fabric is challenging if you are a beginner. You might be better to hand baste with large stitches before you sew. Or pin it well every inch or so all around. Cotton makes a good lining too and is much easier to sew than lining material.

When you get to the corner go slow. Put your hands on the material--one hand flat and one on the edge and slowly turn your fabric as you sew. Or Take a couple of stitches then turn slightly, and take a couple more and continue until the corner is sewn. Again practice on fabric scraps until you get it right

And shorten your stitch length as you approach each corner, lengthening again after you've made each curve.

Hi - you did not say whether you clipped the 'straight' corners or not. When joining a straight piece to a curved or cornered piece, it best to staystitch a hair away from the seam line then clip the fabric so it can spread out to fit around the bends.

For corners you need only make 1 cut right into the point where the corner will be, for curves, you make a series of clips. Staystitching (a line of short length stitching slightly off the seam line & on the seam allowance) keeps the fabric from fraying past the stitching line when clipped.

IMPE, most pattern instructions include this information, but you didn't say that you're using 1.

HTH