A few comments and observations:
Use "Cuillère" (also can be written "cuiller") instead of "levier", the literal translation for "lever". As per definition in weaponry context, a "Cuillère" is maintained in place by the "Goupille" (pull ring), which seems a more accurate translation in current context
Pay attention to accents:
Ne pas utiliser sur où près de substances inflammables ou sur de l'herbe séche
"Séche" is written wrong, it should be "sèche"
Also to the end of infinitive words, often written as imperative
As Upnorth noted, "Ne pas laissez" (Laissez is imperative) should be written "Ne pas laisser" (Inifitive)
About "Throwing Hand":
I'm not a fan of "main lanceuse", although it's probably the most accurate traduction I can think of. I would also consider the use of "main dominante", which by extension would mean the same think as one would use his dominant hand. Note that a (real) quick search haven't resulted on any valid use of the term "throwing hand" in english used as a noun. Is it a really valid expression?
Hold in throwing hand; twist off top cap, turn cap over; strike top of cap against fuse
About the cap: Suggestions were "couvercle" (lid) and "bouchon" (as in "cork" or "stopper"); I'd rather use "capuchon" which is a literal translation for cap and is used for small lids or cover (capuchon de stylo <-> pen cap)
Hope this help