To split long branches using a froe, have a look at the photos page on my website. There's lots of pictures there.
To split a large felled trunk you need to make some wedges. Take a straightish branch about three or four inches in diameter and about twelve to sixteen inches long. Stand it up on a stump and keeping your fingers towards the top, start chopping bits off at the bottom, equally on both sides until you have a nice wedge. Now turn it over and do the same at the other end. You'll end up with a long piece of wood with a wedge at either end. Now saw it in half through the middle. Now you have two wedges.
With the log to be split lying on the ground put one end of it up against a stump. Place your axe on the end grain and give it a whack with a big heavy stick to knock it into the wood. The wood will start to split at one end. Leave the axe in the end and take one of your wedges. Pound the wedge into the crack that has started along the side of the trunk. By pounding the wedge in, the axe should have become loose. Remove the axe and place somewhere safe. Take the second wedge and pound it into the split a little further down the trunk. The first one should have loosened off. Remove it and place further down the split. Keep leapfrogging the wedges all the way down and you'll find the log pops open into two halves after you are past the halfway mark. You can then use the same method to split the two halves into quarters, eighths, sixteenths and so on.
The diameter of the tree being split is only determined by the size of the wedges and how hard you can pound them in. Early man used to make planks that way before saws were invented. The only tool you need for the whole operation is an axe. You can make the rest from what you find lying around.
Hardwood is best for making the wedges (ash being one of the best) and if you have time fire harden them to make them last longer. If you are only doing the one log, then use anything to hand. It doesn't matter if they get trashed at the end, it's not like they cost anything except a bit of muscle power.
Have fun,
Eric