Dig down to expose the roots,which is not very deep,then saw off the roots to free the trunk.I did this ten years ago and found it relatively easy

I have done this as well and the best tip I can offer is to get it done in the cool (ish) weather as it's much more exhausting with the sun beating down on you, having to stop to re-apply sunscreen and get drinks; I learnt this the hard way!

Leave a good length of trunk on as this gives leverage to loosen any roots you arent able to cut.

How many stumps have you got to do? 8" diameters are quite a project! Side roots will be shallow but there is bound to be a deep central tap root. Do they need to come out - once cut to ground they will not resprout. Have you thought of calling in a stump grinder, who will also rotorvate the area? They are not as expensive as people think. If you are in the midlands or surrounding area, send me a private message and I can give you a contact name and number that I recommend.