This is one way I make slab plates. I posted the pictures to FB but am reposting here with descriptions of each step. You can adjust the steps if your clay is more or less forgiving than mine.
First cut slabs and let them rest overnight to stiffen up some. These slabs are 10mm thick.
Place the trimmed slab bevel side down on your form and paddle it THOROUGHLY, from center to edge, then once more evenly all over. You can use whatever you want as a paddle, here I used a sandbag, but I usually use a wooden paddle.
Press the center of the slab down gently and let the edge pieces support the edge of the plate. While supporting the edge with one hand, use the other to define a concavity in the bevel with a convex tool. Anything convex and with a curve you like will work. I used a little ball here. Sometimes I use a rib, sometimes a roller, sometimes a clamshell. I like rollers and balls because they compress the edge well.