To craft a magic item, you must have access to each of its requirements. If you don't know a required spell, you must procure the aid of someone or something that can provide the spell for you during each day of construction. It can be a scroll or wand you purchased, a spellcasting cohort from the leadership feat, a pet with a spell-like ability, or any number of other roundabout ways of gaining the required assistance you need. You still add +5 to the difficulty of the task for each prerequisite you do not meet though.

When crafting, we will not be using the Spellcraft skill for construction purposes. (It is still useful for identification purposes.) Instead, you will need to roll another appropriate skill. For example, you may need to roll Craft: Jewelry to make a ring, or Profession: Scribe to make a scroll. Some items mention a specific skill needed to craft it, and substituting another may be prohibited, or at least incur a +5 penalty to the DC.