The map was easy to read, but Jena was not good at reading maps.
We simply add a comma and the coordinating conjunction, but.
Advanced Study:
Turning these two simple sentences into a compound is only done if that the compound sentence that you create is what you actually want to say. Perhaps the sentences need to be left in their simple forms. Perhaps, the concept that you want to portray would be better served in a complex sentence. Here is an example of that:
Although the map was easy to read, Jena was not good at reading maps.
In the case of creating a complex sentence, we use a subordinating conjunction (there are a lot of them), not a coordinating conjunction.
In the end, you, as the writer, will get to make that decision. But regardless of what decision that you make, you need to follow the rules.