As its been said you can substitute one or two if they have similar qualities and/or you have a closer connection with the item in question [I tend to add cinnamon to most spells for this reason] but if you don't have any of the items, just write your own. Ingredients for spells are chosen for a reason, as well as the number of items [3, 7, 9, 13 are some common number pairings with magick] so if a luck spell calls for 7 items but you only have 6, this could effect the spell so try to keep the number the same if its important.
It doesn't take much skill, but you need knowledge, as said above, knowing the item and why its used is important to cast a successful spell just as visualization and energy is important. Before casting a spell, try researching the items to understand why they are in the spell so when you use it you can reflect on why it is needed.
If you are missing one thing you can substitute it, like you don't have a blue candle, white works, or you don't have a wand you can use an athame. For herbs, crystals or incense I would find one with the same qualities [luck, protection, clairvoyance]