Step one is to cut four pieces with the long edge on the fold, 2 out of the main fabric, 2 of the lining. Wait, step one is to make sure your kitty knows you love and adore her. Then put her in the hallway and close the door.
Okay, first step: With right sides together, sew a seam up each side of the main fabric, then do the same with the lining fabric. Iron the seams, and position them so they run down the center front of the bag.
Line up the center seams and the top edges of the fabric and start pinning the main fabric to the lining. You'll end up making one continuous loop, up one side of a handle, down the other, then up the other handle, etc.
Once it's all pinned, you sew along that same edge. Start at the center seam, go up and around one handle, etc. It's all one seam.
Now take the time to clip the curves. Blech. I hate doing that, but you really should.
Once that's done, trim along the seam a little if you think you need to, then turn the bag inside out and sew another seam, this time a half inch, across the bottom, which means the raw edge is encased in the seam, aka a French seam.