I just did this the other day. I had the same concerns after reading the DIY in the STAR magazine. You break off the red tab and with a small screw driver, push the remaining part of the tab down and it falls outside the fill tube. Lucky for me I found this piece later as I will explain. I don't believe you can take off the fill cap with this part still in it but it is only a small piece of plastic. It may be that you can just muscle the cap and remaining tab off. You can experiment as the tabs are only $1. Were YOU able to get it off just by yanking on it? The fill cap secures itself by pushing down on it and it will click in place. It takes a bit of pressure. At this point, with a tug, you can just pull it back off again. After you add a new red tab, the fill cap is on until you break the tab. No real need for the red tab unless you are afraid that some indie might mistaken it for something else. What I did wrong was not push the cap fully down, so when I added the new red clip, the fill cap came right off in my hand again. Looking at the old tab piece, I figured out how to remove the tab without breaking it by pinching it from underneath.

glenmore
1991 300CE
1990 LS400
2000 C280