My lengthwise scarves are curving! bind off loosely?

I’m a beginner. Having made a few scarves lengthwise, I’m seeing a consistent problem. I’m binding off too tightly, and the scarves are ending up with a slight arc to them.

All the instructions I’ve seen say “bind off loosely”. Can someone give me some practical advice on the best way to do this? Should my bind off stitches be twice as big as normal? Is there any secret to managing the tension in the two stitches before I cross one over and drop it off?

I’d love to know what specific tricks and techniques you use.

had a problem with the bottom of the scarves being bigger that the rest of the scarf. started with needles 2 sizes smaller that what i wanted to do the scarf in and then changed to the larger size after 3 rows. seems to work for me.