This topics been covered a lot, your best doing a quick search for it. There’s no need to take off the grill though.. easiest solution is red 3M tape. Do a search anyway for the threads that have photos and examples.
6mm pin stripe from eBay. It's what I used. Some IPA to clean the grille and take your time. It's not a hard job. I think the tape cost me £3 delivered.
Nope, leave the grille on the car. Make sure you clean the area you are going to apply the trim tape with pure alcohol (IPA is what I used) so that the tape sticks. The trim tape should be self adhesive, heat the grille a bit with a hair dryer if it's a cold day as heat will help adhesion. Then simply take your time and apply the tape onto the grille (you'll see the smooth edge that it goes onto). Took me about 10 mins start to finish.
Cheers paddy this sounds easy enough but I've actually bought a red trimming rubber which has a rounded edge and flat end that should be wedged in between the grill and surrounding edges so it may be a whole different ball game for me. Can you upload a photo for me to see yours please ?