Process I use.
Start with bare bumper without headlight buckets, grills, headlight doors (if equipped).
With frame horn brackets off of the car, test fit bumper to car and make sure it aligns to the fenders the way you want. You need at least one helper for this step.....preferably two. The two helpers hold the bumper in place while you inspect the fit. If you are unable to get it to fit well at this step then you need to correct the issue whatever it may be. (Fenders not aligned properly, out of square, one too far forward or too far back, bumper bent or mis-shaped, etc)
If test fit passes, then attach frame horn brackets to bumper with bolts slightly loose. Have those same two helpers offer the bumper up to the car and hold it in place while you install the frame horn bolts snug. Protect the painted finishes on the fender and bumper.
Separate the bumper from the frame brackets and load the bumper (install headlight parts and grills).
Reinstall bumper to frame brackets and final adjust to fit. Torque fasteners as required.