No idea about the DIY route, but you'll need to pull off the existing caps and then screw on the spikes..
My Gitzo tripod seems to have an internal thread where the spikes just screw into it and have the rubber caps over them. May be worth checking your Manfrotto to see if it has the same?
Spikes for Manfrotto are about £20 for a set IIRC
EDIT: if the Manfrotto isn't the same, may be worth in measuring the diameter of the Manfrotto bottom tubing and buy some threaded sleeves that tap in and get some Gitzo spikes with the rubber cover. They're £20 for a set as well..