"To get a neutral BW image, convert the image to grayscale first. Then they can convert to a color profile such as sRGB using Relative Colorimetric intent. This will achieve true BW based on the white point of the paper."
Can anyone break this process down for me in detailed, but, layman's terms?
They are telling you they want sRGB (the bit about the rendering intent is moot, you're always going to get a Relative Colorimetric intent). Then as mentioned, Desaturate the image (Hue/Sat, the Desaturate command etc). As also mentioned, unless that lab has a really good profile and process control, it's kind of likely you'll get a print with some color cast unless somehow they are imaging this C print onto B&W paper.