If I could shrink to ant size, how could I do this?

As I said before, you couldn't survive a month inside a spider given those terms, if you shrink using a shrink ray and nobody will help, you will be stuck that size forever, so I suggest trying to get this neighborhood girl to forget or something that won't involve you getting caught in that because given those terms, you are automatically dead. If she shrinks you, try to escape, or after being fed to the spider, but if she monitors that and by some miracle you survive the month in the spider, free yourself from the poop quickly and find an exit fast, then run as far as you can, find someone that can help or live with that size, because shrink rays may be developed in the future. Being that size, life would be very dangerous because you'd be food for many animals and most things are a hazard so you should hope that girl forgets the deal or no shrink ray is built.