Cutting hand knitted fabric?

Knitting will unravel so you need to mark the place where you cut then stitch within the seam allowance before you cut. Using the zig zag stitch on a sewing machine is good for this or use an overlocker. Machine knitters knit lengths of fabric and use them like dressmaking fabrics all the time. Google "knitting cut and sew" for more details.

If you link a picture of the character you want to cosplay, perhaps someone could suggest a knitting pattern that you could adapt.

I could tell you the advanced way of steeking (cutting your knit fabric and finishing the edges by hand), but I doubt that you would want to put that much work into a costume piece.

I don't know what cosplay costume you're attempting to make, but I would suggest searching thrift stores for cheap machine knitted ponchos, shawls, or blankets, and using them for your costume fabric. You can dye them if you can't find the right colors, but that's another question.

I'd draw or outline my pattern pieces on the knit fabric, and then use a decent sewing machine to make a tight zigzag stitch along all the pattern lines (like a buttonhole stitch completely binding the fabric around the outside of each pattern piece). Then cut the pattern pieces just outside the zigzag lines. You could then sew the pieces together by machine, or whip them together by hand.

You would knit the size and shape you needed. Then sew them together with the same yarn. My daughter has made sweaters and dress that knitted by hand.

To cut through hand knitting you would have to surcure the edge before you cut it.