Carefully read any submission guidelines. Some companies will want a complete project, along with precise instructions. Others may only want a photograph or chart of the design. Most will want a complete materials list, along with instructions, and at least a diagram, photo, or chart, if not the completed item. Following the guidelines may be the difference between your idea actually being reviewed for possible purchase, and being returned. Think professionally, and assemble what you are going to submit in such a way, that the editors know you both read and followed their guidelines.
Detailed Instructions
When writing the instructions for your idea down on paper, be as specific as possible. Write detailed steps as you go, and do not rely on memory to draft it all at once when done. Skipping a single step, even a very basic one, can cause confusion for the intended reader.
Think of how patterns are displayed in the magazines you read. Do not try to find your own style when it comes to explaining how to create something, unless there is no other way to do it. Most crafts have a very specific way that their instructions are written. Consider crochet; most crochet patterns are written with abbreviations for the different stitches, and either written out in rows or rounds. If you stray from this normal pattern, not only would it be hard for a reader to understand, an editor will most likely reject it instantly as being unreadable.
If the publication request photographs of the item, take a series of photos during the different steps of assembly. Label each photo in correspondence to the accompanying instructions.
Ideas and Copyright Infringement
No one can teach you how to create a unique craft item. What you can be told, is that you should never copy any part of an existing pattern or craft. Copyright infringement is serious and is never taken lightly by publications that you may be submitting. For you to submit a chart, pattern, or design for publication, it must be an original.
Where can you get ideas and inspiration? You probably have more ideas floating around in your head than you know. Keep a notebook handy at all times, and when something comes to mind, write it down, or sketch it out. Keep up to date on new craft products. Oftentimes new products can be inspiration. Subscribing to current magazines will also keep you aware of current and popular publishing trends.