Select one Line, press right mouse button. Search Convert and then Click Edit poly. Now go to the command panel, select Modify (the one next to the mouse pointer. You still have one Line selected) Scroll down and search for Attach. Press next to Attach. Then with Shift (hold it) and select al the Line's. Press button Attach.
Are we there yet? Almost
Don't leave the command panel. Press 1, its located on your Keyboard. Now scroll down in the command panel, search for Quickslice. Look to the image and you will understand where to place it! Do it on both sides
When you done that, Select the vertexes, left and right from the slice we just made! And delete those
Ok this is cool. But I want a curve in it! Let's do that, go to the command panel (Modify, the one next to the mouse pointer.) Select your grille. and in the command you will see, Modifier List. Press it, and you get a whole list with stuff, search for FFD 4x4x4. Now you will see Editable Poly and FFD 4x4x4, Press the + in front off FFD 4x4x4. You get 3 options now, select Control points. And now make that curve. Do it only with Fully selected left/right side. Or those two Fully selected middle.
Tweak it to make the best curve, not like this one above.
If you followed the tutorial above complete. You can get this as a end result
Good luck! Thank you for reading this tutorial