Take a piece of twine and tie the plastics to the bumper - make a hole in the bumper and plastics so the twine can weave through it similar to how you would lace your shoe. Afterwards, clean it up so the twine is does not show. You may even want to think about using a black rubber gasket on the plastics side so it doesn't tear.
Here are more pictures of that...https://www.celicahobby.com/photos/celica/installs/inst_wheel-well-plastics/
If you're talking about extending the bottom of your stock wheel well plastics (if you have already cut them "straight") to meet the bottom of your blitz bumper, you could take a dark colored sign (or paint a for-sale sign) and bolt (or attach it with glue) it to your stock plastics (it will look "ghetto") similar to whats described here...https://www.celicahobby.com/forums/...Intake-CAI-Fender-Wall-Liner-Custom.html
But I believe that tying them with twine may be the best solution. Also, check out this post...https://www.celicahobby.com/forums/...you-taken-this-off-If-so.html#Post448960