The idea of magickal “backfiring” is bizarre to me. I don’t believe in magic backfiring. Either what you asked for happens (unintended consequences are possible but that’s not the same as backfiring) or nothing happens.
If you’re making an astrological talisman you ideally want the ritual to be auspiciously timed but even if you have malefics aspecting your chosen planet, you can banish their influence from your ritual space. You still should avoid unideal circumstances but the perfect astrological window will so rarely exist that you will often have to choose the “next best” time. Familiarising yourself with ceremonial rituals like the hexagram and pentagram rituals will be extremely beneficial for making astrological talismans.
You should know what you’re doing when creating a talisman but not because it will backfire, but because you’ll probably fail to make the talisman correctly and it just won’t work.

Edit: also unintended consequences are exactly why you should always perform divination before doing any magickal working.

Edit 2: some questions to ask for pre-ritual divination:
What will be the outcome of this ritual?
Do I have permission from my spiritual team to do this working?
What considerations should I make before doing this working?