So basiclly you want the hood and both front fenders to become one piece.
PLEASE READ ALL OF THIS FIRST BEFORE CUTTING / GLUING ANYTHING!
Before cutting: Place a piece of wood behind the fender/door. Dont glue it there...just hold it there. you may want to use some clamps to do this. If you use clamps dont tighten them down too tight. You dont want to crack the plastic. The wood will help keep you from "Mauling / slashing the car body , and hopefully YOUR HAND !
Since the hood is ( in most kits ) already seperate you need to cut the fenders off at the line where they meet the door. I use a SHARP X-acto blade and make several "LIGHT" passes down the seam. Then you need to use some strips of plastic along the underside of the hood where it matches up with the fender's and glue them in. This is for Extra support. You may also need to mount some sort of bracket/support to the firewall to keep the hood from folding too far in.
PLEASE test fit first before you glue..... sometimes parts need to be checked out for fit issues. They may need to be sanded. Once you get the hood/fenders assembled let the glue dry THOROUGHLY before tackling the grill/ headlight's.
On a normal car there is usually a mounting ( sheet metal bracket/ wall across the front ) for the radiator. sometimes the battery is mounted to this too. If the car your working on has this already moulded to the fenders LEAVE IT THERE !!! It will help keep things straight. If your car doesn't have this you will need to make one out of sheet plastic. Plus when its time to mount Hinges to the Flip-front , this is probably where they will mount to. But it depends on where the frame lines up so you will need to look CAREFULLY and think about where they will actually go. You may need to add some strips of plastic as spacers to get the hood/fenders to line up with the frame. REMEMBER: the hood/fenders must also line up FLUSH with the rest of the car when the flip-front is closed.
Once the flip-front is working to your satisfaction, then you can tackle the headlights, Grill, front bumper. The bumper may need to be extended slightly from the front of the car.Because we have "Altered" the hood to tilt forward there may now be an issue with the bumper getting in the way of the hood flipping all the way forward. Again, test fit.... Imaginerring is kind of a hit and miss procces so take your time. Usually the bumper can just be attached to the hood/fender assembly just like normal. But you may need to add small "Shims" behind the bumper to bring it slightly forward. If the bumper mounts to the frame ,this is another good place to put the hinges. Test fit
Once all of this is done you will HAVE to make a new mounting for the radiator ( radiators dont like to be flipped ) A simple frame around the radiator will mount it to the frame. AGAIN test fit first. Temporarilly put the motor in the car so you can figure out where the radiator should be mounted.
There will be some "leftover" bits of plastic here and there that you will need to trim off/ sand smooth.
Since I dont know what car your doing I cant really offer any better advice.
Let me know if you get stuck and I will try to come up with a solution.....
( sometimes I use a HAMMER...... and start over.... )