Select the half of the untouched part of the bumper you've modified (say that you added an intake in the right side and you want to mirror it to the left, so select the left half part) but DON'T select the vertex at the Middle plan (the vertex with X=0 coordinates)
press delete key to remove them
SOLUTION 1: separate the bumper half modified part by selecting the concerned vertex/faces and press "P" key, then apply a Mirror modifier to automatically spawn the symmetrical half part. Condition: Bumber object center (object's local origin) must me at X=0. Select in the mirror modified panel the correct axis to get the desired copy direction (try them). And adjust the merging radius if needed because the middle vertex are duplicated to the exact same position so this feature automerge the vertex to avoid a unwanted medium line.
SOLUTION 2 (delicate but more reliable): place the 3D cursor at X=0 (Y and Z don't matter) and select "3D cursor" in pivot tool selector on the left of the hand icon of the manipulation tools. Select your half part of your bumper EXCEPT the vertex located at X=0 to prevent duplicate them to the exact same position. Press "shift+d" to duplicate and immediately after press "S" key for scaling the new created duplication. Then press "X" key and type "-1". The duplicated half bumper part should be now mirrored, its scale should not be altered and be symmetric along axis X only with the symmetrical center as the 3D cursor.
Problem: check the UV mapping. Both bumper halves shares the same UV half part...
OK, when you open your scene, you see your car etc...
You know where the layers are, dont you?(They are beside the roatating tools) If so, click on the first little box (ie the first layer). All you will see is the car.
Now, press 'a' untill everything is selected. Then press 'shift+d' and click the left mouse button.
Next, click "object" and go to "move to layer", then you can move it around etc..
Then you select all the active layers and Render!!