For the vehicles take 9007 or 9004 bulb, some of them have a retainer/locking ring, which would secure the original bulbs into the headlight assembly. Normally, the retainer ring has two types, here are the images of them:
If your retainer ring is type 1, while installing the LED bulbs, you need to remove the white collar on LED bulb(turn it to the open then slide it off), and install the retainer ring from the front, then put the collar back on.
One of the customers followed our instruction, and installed the 9007 bulbs on his 2003 Ford Explorer Sport Trac within 10min, what a great job!
However, if the retainer ring is type 2, a little bit longer than type 1, then you have to modify and cut the retainer ring, to get it fit into the LED bulb.
Here is what the customer Andrew did for his Ford F150 1991.