How do I redirect a downspout and properly seal off an old, unused catch basin in my yard?

Redirecting a downspout is usually an elbow and extension, or tying into a new, properly sloped drainage line away from your foundation. For an abandoned catch basin, you need to disconnect it from any active pipes, clean out debris, then fill it with gravel and cap it with concrete to prevent collapse and water pooling. If the basin connects to a municipal storm drain, you might need a permit to disconnect it properly and abandon it.