Some people have problems making map item signboards, as they will be overwritten if you go near a certain point on the map (usually X:0, Z:0). There is an easy solution: In 1.14, cartography tables were added, which allows you to clone, zoom, and lock maps. The latter is especially important: Locking a map means it cannot be overwritten. Here's how you can make a locked map item signboard.

1. If you are near X:0, Z:0, move to a place far from it.
2. Use the MC Map Item Tool to turn an image into a map.
3. Use an NBT editor (like this one) to add a byte tag. Name it "locked" and give it a value of 1. OR
   
4. Use an ingame cartography table with a glass pane to lock the map.