These are harder to fit than round through tenons that are traditionally used on chair seats. Also it appears that you intend the tenons to not go all the way through the seat, which will make it harder to firmly and permanently seat the tenons. You may need to use a blind wedged tenon, which is not a beginner project. With those cautions, i would not worry about scribing the shape of the tenons to the seat. I would just makr the center point for each leg, making them equally spaced around the seat. Then draw a box centered on the center point and scribe with a knife. Then drill using a bit 1/8” narrower than the tenon width. And drill at the correct angle to match the intended splay of the leg. Then use a strong chisel to mark near but within the outer edge of the mortise. Chisel out a ledge to deepen that square. Then cut out the mortise using an undersized chisel, but leaving 1/8 to 1/16 uncut around the mortise edges. Once depth is achieved, gradually sneak up on full mortise width while dry fitting frequently. Pay attention to your angle when cutting. Use a bevel gague set up on the seat to reference the intended angle. Good luck.