Hi, Kaitlynn - your question is important and I am sure many with better photography skills than I will respond. But here are some thoughts:
Carefully consider the condition of each. Because they may have been handled extensively (depending upon their source, previous "use," and storage conditions), they may be creased, weak at the edges and torn from battens.
You may find digital COPIES of them online. If they are 19th c. insurance maps or semi-perspective aerial views, for example, they may be digitized. In such cases, you can download the digital version for your catalogue (citing, of course, the source of the image).
If that's not possible, even though it may seem time-consuming and staff- (volunteer?) intensive, gather a group of people you can train about handling unframed "works on paper," unstretched "canvases," fragile textiles, etc. After training, assemble the team in a sort of assembly-line approach.
Be sure to have on hand weights and tissue-wrapped battens. The weights you can make yourself or purchase sandbags from University Products or Gaylord. Find a large, open space (a gallery when the museum is closed?) then set up your photography equipment, to include a good, sturdy ladder, clamps and extension pole (boom). The latter you can find at B&H. If you have stairs that are open and overhang an open space, even better. Lay out a clean sheet on the floor below the camera.
Your team will need to be able to crawl around on the floor and yet be very mindful of care for the documents. Have at least one 8' table nearby to set the maps on before they are brought to the sheet and carefully unrolled (or, in some cases, unfolded?) As you unfold, lay the weights strategically to keep the map flat. Lay the battens carefully along the side edges also to keep the map flat. Do everything as if moving in slow motion. Then reverse everything, again, very slowly.
Is there a University nearby? If so, there may be history (or even better, museum or archives studies) students who would be useful (especially crawling around on the floor!) and they may earn credit for the time. again - training before is key.
Good Luck!