There is no set or cut and dried plan for a test garden. Basically all you need to do is dig holes and put in your test objects at varying depths. Also a good idea to plant several coins so they stand on their edge as these can present difficulties detecting so you want to know how your detector does on these. Also plant some trash types.
You can either mark the holes where you planted your garden, but one thing you should do is draw a map of your garden so YOU know where in your garden, what is planted.
Some folks place a rock to the side of each item that is planted, others will put a stake. Others just draw the map and depend on that and memory.
Also recommend in one spot in your garden you plant 3 or 4 coins fairly close together, as in a couple inches apart and another one or two several inches away from the small grouping ub seperate directions. This lets you see how a group of coins will show with your detector. And maybe add a piece of trash in the grouping. Or do a seperate grouping with a piece of trash in it.
One thing to make sure you do, before you plant anything is scan your garden with your detector on all metal mode so you make sure your garden doesnt have any trash or anything else in it other than what YOU want to plant.
Hope this helps.