Step 1:Know what you want to replace and get a model. In before making this tutorial I ate some French cheese, so I decided to make a baguette, or stick bread.
Step 2:If you have your model in blender, you have to put a texture on. The most basic way is to first split your screen, you can do this by right clicking the black border in Blender, like so:

After this you set your second screen to UV/Image editor, like so:

Now the most basic way to set a UV map is to tap A one or two times, untill all the vertices in your model are orange (Make sure the mode in the model window is in EDIT MODE). After that you can press U, and select 'project from view'. This will put an image your model onto the UV editor. You can then save the layout to use in photoshop:
(This is the most basic way to UV map your model, you can do it way more accurate, but just look some tutorials up on that.)

After you made a texture for your model you can test it in Blender, by going to image>open Image in the UV editor. Also make sure you put the drawing mode in the model window to 'Textured'.
Step 3:You now have to position your model. Import an original silkroad model into blender and move your weapon untill the part where you hold your weapon is the same as the part where you hold the original weapon (I put my window on wireframe mode now, to see it better. (DO THIS IN EDIT MODE, NOT IN OBJECT MODE, the pink dots of the new and the original mesh have to align)
Step 4:Once your model is aligned, you can export it, make sure to select your model in object mode, and use the export settings as given above.
Step 5:Before we can convert the model, we have to find the model group and the material name of the model. Take the file of the mesh you are going to replace, and open it in a Hex editor such as XVI32. You are looking for two words at the beginning of the file like this:

Use these in the converter, and convert your model, if everything went right it should say so in the log.
Step 6:You can import your mesh into the data.pk2 now. Make sure you overwrite the right file. Also dont forget to save your texture as a .dds, and convert it to .ddj. Then replace the texture with the same name as the material name of your model with your own texture.
If you did it right it could look a bit like this: