Shaders are one way to do it, but that depends on the total effect you are going for. If the only change you are interested in is to change the "visibility" of the model then you can use a BasicEffect instead of writing a shader. Just turn on the Alpha Blending render state on the graphics device and set the alpha value. For Alpha values 1.0f is fully opaque and 0.0f is invisible - you probably want something closer to zero.
Example (Modified *very* slighty from the XNA documention on how to draw a model):
private void DrawModel(Model m, Matrix World)
{
Matrix[] transforms = new Matrix[m.Bones.Count];
m.CopyAbsoluteBoneTransformsTo(transforms);
foreach (ModelMesh mesh in m.Meshes)
{
foreach (BasicEffect effect in mesh.Effects)
{
effect.EnableDefaultLighting();
//you can modify this models effect here
effect.Alpha = 0.5f;
//modification done.
effect.View = m_viewMatrix;
effect.Projection = m_projectionMatrix;
effect.World = transforms[mesh.ParentBone.Index] * World;
}
mesh.Draw();
}
}
