If you have a nice pond, you don't need to do much. The goldfish will spawn on their own when they are old enough. Adding soft plants like anacharis, floating plants with soft roots like water hyacinth, and marginals will give them soft places on which to lay their eggs.
My section on sexing goldfish is at
http://www.fishpondinfo.com/gfish3.htm#sexA shelf won't do much good. Both parents must be able to go wherever they spread their eggs, preferably some place soft. Once the eggs hatch, if not eaten, then having nocks and crannies in which the fry can hide will improve their chances of not being eaten. Adult goldfish will eat most eggs and fry that they can find. Luckily, they lay so many that a few usually survive. If they all survived, all our ponds would be beyond capacity from only a portion of a single spawning. If you want more fry, you can remove eggs or fry to smaller ponds to grow larger.
Females may be injured during spawning if they are pushed into rocks, etc. so remove sharp-edged rocks and other things. They may also try to spawn in filters if they can get in so prevent that.
Goldfish tend to first spawn when about 3 or 4 inches long but aren't really mature until a few inches larger. I've had adults that were 14" long.
The toads won't eat the eggs.
Good luck!