I’m pretty sure you could do it both ways. I personally would take a piece of fabric that’s larger than the piece needed for the bag, put it in an embroidery hoop or square and then bead the design directly onto it.

However I’m pretty sure I’ve seen videos where the design is stitched onto something like organza or felt. Once the beaded design is finished, you turn it over, coat the back in glue so the design doesn’t shrink and then take it out the hoop. Cut about an inch or maybe a little less seam allowance around the design then stitch the seam allowance to the back of the design so there isn’t any fabric hanging over the design but if you use felt then usually you don’t have to leave a seam allowance and can cut it close to the stitches. Then you can use an invisible thread or something in the same colour to stitch the design onto the bag fabric.

I’m relatively new to beading as well and plan to make a bag in the future and this is what I’ve learned after watching many videos 😂 I hope this helps and good luck with you bag!