I think people are going about this the wrong way. Geese do an amazing job at weeding a garden - even a small garden - you just have to be smart about it.
We section our garden off into plants that are safe for the geese to be around at any given time. They do great with tomatoes, but once they know what a ripe tomato tastes like, they can't be kept there any longer. Until then, let them weed your tomatoes for you. They do a great job with corn, but again, you have to have them in the corn area at certain times. Young corn plants are going to be gobbled up, but larger, established plants (you know, the plants that have grown large enough for you to give up on weeding and just accept it's a lost cause) love geese.
Potatoes are completely safe for geese. They don't dig up the roots and ignore the leafs. Onions are also safe.
I do things a little differently with my weeders and bug munchers. Instead of letting the adults into the garden, I keep my goslings _and_ my ducklings in the garden. The ducklings will run around munching on bugs, ignoring the plants. The goslings will happily uproot any sprouts they come across. They are small, so there's no worries about them trampling anything. Last year I had over 100 ducklings running the garden and 50+ goslings.
This year I will be tweaking my design so the adults can get in there, too. I have 4x8' plots in my garden. I will be running cattle wire around the outside so the adults can reach into the patch and munch on the weeds, but not actively climb inside of it. I lose the benefit of having their manure in there, but don't see that as a big deal.
You don't really "train" geese to weed a garden. You set up your garden for your geese and reap the rewards. That is a lot less work than standing over them with a stick or bending over to pluck the weeds yourself.
I have an article about it up at
http://omniskies.com I have no idea when I last updated the article, but it should still have some decent information in it for anyone interested in using weeder geese.
