Creating a Costume Bible
Let's start from the beginning. A costume bible is an invaluable book created by the costume designer for a show, movie, etc. In more professional theatres, the assistant designer makes and updates the bible. It is a compilation of ALL paperwork and information needed to costume that particular show or event.
You should get started with your bible as soon as you are hired. Get a giant binder and a ton of clear plastic page holders, along with a pack or two of dividers. The dividers with folders are the best. There are many options for what to label each section as or even how many to have. So I'll leave that up to you To start though, I'm sure you'll have a cast list, and a scene breakdown, either from stage management or one you created yourself when reading the script.
The first thing you'll do is make your costume plot. A costume plot is possibly the most helpful piece of paperwork that you'll have. It's a chart of the costume changes in the show. You can make one either in Microsoft Excell or in Spreadsheet. Across the top put each scene number, one per column. Going down the side, enter every actor's name. If helpful you can add the character's name for each actor and a brief description such as age, rank, job, etc. Same goes for each scene. You can add a brief description of each such as location, time of day, any important action that goes on, etc. Then go ahead and put an X in the appropriate boxes, marking what scenes each actor is in. Also write in any wardrobe notes that were in the script. These include taking off a coat, putting something in a pocket, ripping a skirt, etc. Anything that you need to know when selecting costumes. It's also a helpful idea to number each costume change. That way you know when they stay in the same clothes scene to scene and how many costumes you acually need. Once you have this basic outline done, you'll have a much easier time keeping track of what you need. As you assign costumes you can write them in the empty boxes.
Something else you should do early on is research. Lots and LOTS of research. You need to know that time period backwards and forwards. Generally, you should research that year and season that the show takes place in, along with several years beforehand. Google images is great for this type of research. For twentieth century clothes, check vintage sites. For older than that check museums. The paintings are very detailed. There are many books available as well, including some with actual patterns from the time. I'll put links to some at the bottom of the page. When you find really helpful research, make copies of the pages and designate a section of your bible just for them.
As soon as the show is cast you need to have measurements taken. If there aren't any builds in the show you can just take down the actors' clothing sizes, although I recommend still getting waist and chest measurements since sizes differ between brands and styles and periods. Make a section in your bible for all those charts. For easier reference you may want to create what's called an "At-A-Glance". It's a chart with the actors' names down the side and key sizes or measurements going across the page. That way, if you're shopping and don't want to carry the whole bible, all you'll need is the At-A-Glance and a tape measure.
If all you're doing is shopping for costumes, you should make a section of the bible for receipts and budget. Create a budget breakdown by approximating how much money you anticipate spending on each costume. Then, as you shop, keep the receipts in that section so you have them to copy and get reimbursed from the production manager. If you lose those receipts, you just spent your own money on the show and no one wants that.
If you have builds in the show, you need a section of the bible designated towards materials. You should include a copy of the sketch, and swatches of each fabric with information about purchasing it such as width, price, store, and how many yards were bought. Also include, if needed, information about zippers, buttons, and any other add ons required to make the costume. This is helpful for budget reasons and for the shop making the build. Also for long running shows, the bible stays in the shop and when new costumes are needed they know exactly where to buy the fabric and how much to get without wasting time searching through stores.
Speaking of sketches, you should definitely have the sketches or at least copies of the sketches in a section of the bible. Some other helpful things are to take pictures of the actors in their costumes in the fittings. That way you remember every detail of the costume. You also may want to make an itemized list for each actor. It will say everything they are wearing from shoes to belts to necessary undergarments to jewelry for each scene.
Creating a bible may seem like a waste of energy but I can't stress enough how helpful it is. Designing a show is stressful and sometimes you can get distracted or confused. But by being organized and keeping everything you need to know in one binder, you minimize the amount of time spent looking things up and can just focus on what you need to do.