How to create Mock to a class that implement an abstract class

I have the example above and i need to decouple the code to ease the tests, how i can do this to can test using mock?

public sealed class XCommand : BaseCommand
{
    public IResult Author()
    {
        //DO SOMETHING AND RETURN BASE.

        return base.run();
    }        
}

public abstract class BaseCommand
{
    public virtual IResult Run()
    {
        ProcessStartInfo startInfo = new ProcessStartInfo();

        using (Process process = new Process() { StartInfo = startInfo })
        {
            process.Start();
            process.WaitForExit();

            String error = process.StandardError.ReadToEnd().Trim();
            String output = process.StandardOutput.ReadToEnd().Trim();

            return new Result()
            {
                Error = error,
                Output = output
            };
        }
    }
}

sealed class Result : IResult
{
    public String Error { get; set; }

    public String Output { get; set; }
}

there are a lot of class that will inherits the class BaseCommand and i need to test every class individually, so how i can decouple this code to be possible make this?

Thanks