OK..
I will give an example step by step to replace an existing model, I will use the F-CA as an example and will be replacing it with a Battlestar.
1. Configure Windows to where you can see known file types. To do this, open up My computer to a window, click on Tools and select Folder Options, click on the View tab at the top, then in the lower box you will find Hide extentions for known file types and uncheck it. click OK.
2. Download the model you want, mine is a Battlestar.. the file I download is BSG.zip
3. Extract BSG.zip to its own folder.
4. Look inside the BSG folder and make sure that there is not a folder inside of it. If there is, then go inside the BSG folder again. (BSG folder inside of BSG folder can happen easily.. you only want 1 folder with *.mod files and *.bmp files)
5. Inside the BSG folder you should see a bunch of *.bmp files like BSG1_Top.bmp and so forth.. what we want is the *.Mod files. so find the BSG.mod file and the BSG_brk.mod file.
6. Open in a separate window the fca folder located in the \Assets\Models folder. Look at what the fca files are. You will see a fca.mod file and a fca_brk.mod file. You will want to rename the BSG.mod to fca.mod and then rename the BSG_BRK.mod to fca_brk.mod
7. go back to where you can see the folders of the ships in the assets\models folder. Rename fca folder to fca1. This will be your original Model backup.
8. go back to the BSG folder that is Holding the *.mod and *.bmp files you just renamed. (Not the BSG folder holding a BSG folder.)
9. rename the BSG folder to fca
10. Cut the folder you just renamed to fca from its location and paste it into the assets\models directory of your game
11. launch game and select the Federation, and choose the F-CA ship and make sure model works.
12. if model does not work, then there may be a problem with your graphics card and / or memory, or you did not label the files correctly to match the originals exactly.. Now you can recheck your files by going into the fca1 folder and matching them to the ones in your added fca folder for spelling and case (upper case or lower case).
You should now be able to see your added model in the game now.
If you want a step by step on adding a new ship to the game, then that is a bit more complicated but still somewhat easily done.. However You will have a problem of using it online as it requires modification of the shiplist and will CRC check as bad when trying to join a server or a GSA match.
You should have no error on replacing a model that already exists in the game and playing on the server