It's wonderful that you want to take the eight precepts, if even for a single day. Taking (and keeping) vows is such an effective way of generating great amounts of merit. So I rejoice in your intention. Even just thinking about doing this generates merit.
As for worrying about doing it perfectly, you can simply say that, under the 8th precept, you will avoid alcohol and so-called recreational drugs and that you will decrease
your consumption of nicotine and caffeine. IOW, you are the one taking the vows and you can decide which vows to take and how strictly to vow keeping them. The merit earned may not be as great as if you could avoid caffeine and nicotine too, but something is better than nothing. You will still be generating a huge amount of merit. If you dedicate that for the sake of universal Enlightenment, then that merit will be multiplied infinitely and it will also become inexhaustible. As the old saying goes, don't let the perfect become the enemy of the good.
After you have completed this one-day set of vows, you might try progressively cutting down your intake of nicotine and caffeine so that, next time you want to take the eight precepts, you can take them completely according to the FPMT guidelines.
Good luck and best wishes.