This question is difficult to answer without knowing exactly what your finished unstarched piece looks like. Even with a lot of "extra" stitches in each row you should be able to eventually create a flat piece.
I haven't made snowflakes in years so I don't remember the exact process but if I were to do one today I would probably do something like this:
- Wet the finished piece with water
- On an ironing board (because the cover/padding is soft) I would lay the damp (or wet, doesn't matter) piece out as flat as I could and then using straight sewing pins I would start stretching and "anchoring" all the the edges by poking the pins down into the ironing board padding. If you have a fancy ironing board cover you might want to rethink making pin holes in it but a fabric cover is okay.
- Once your piece is stretched out and evenly anchored with pins let it air dry (it should hold its shape).
- Get your iron and turn it on to a heat setting appropriate for the type of yarn/thread you used, hopefully your thread can tolerate at least the lowest steam setting. You will be pressing the snowflake before starching.
- Now with your iron heated up (and hopefully steaming) carefully remove the pins from one point only of the snowflake and gently press the iron onto that point. You might want to use a thin cloth or pressing cloth between the iron and the snowflake.
- Repeat for all of the snowflake points and gently press with the iron. You should now have a nice flat snowflake. Allow it to fully cool.
- This is point where you mix the starch. If I recall I used Lacy's Stiff Stuff (available at U.S. craft stores). I would think that Mod Podge could also be used but I haven't tried that. The consistency should be just thin enough to soak into the fiber without gooping up on the surface. You want it to be able to penetrate but not puddle too much underneath.
- Carefully place the flattened, cooled snowflake on a piece of waxed or parchment paper and carefully apply the stiffener/starch with a craft brush, allowing the starch to soak into the fibers of the snowflake. Get as much stiffener as you can to soak in without it pooling underneath.
- While the stiffener is still wet I like to add a bit of superfine iridescent glitter because I'm crafty. Allow the snowflake to fully dry.
- Carefully peel the snowflake from the waxed paper and admire your handiwork!
That's just one way to do it. As I was typing I realized that you could skip the ironing step entirely and just soak the newly-crocheted piece in the stiffener until it is saturated then place it on the waxed paper and "coax" it into shape as it dries with your fingers and/or pins as above.
Make sure you use waxed (or parchment) paper between the snowflake and the ironing board (or other pinnable surface) if you're using pins. Let it dry fully and you have the same result.
I have used both methods. Going through the ironing steps at least let me know what the finished product would look like and if it's even going to look good before wasting stiffener.