You can transfer hole locations either way by using a piece of paper and a pencil ad use the holes in the plate to trace locations for the handle or vice-versa.. If you have access to a drill press ( pillar drill) you have a better chance of getting accurate holes. Keep in mind that with many saw nuts the hole you need to drill on one half of the plate will be bigger than the other side. I drill trough with the small bit and without moving the handle switch to a larger bit and drill halfway. The same applies if your saw nuts need to be recessed.
If you are starting with a predrilled handle, just put a piece of cardstock or cereal box the shape of the saw plate and insert it in the slot. Then prick the hole locations with an awl.
For your first try, do one hole at a time, using saw nuts in the holes you have already drilled to register the next hole.
To cut the saw plate slot, you could just cut to a centered line with a backsaw. I’m sure Paul’s would. If you don’t quite trust yourself, clamp the handle flat to your bench and make a shim to lay a saw on that centers the saw and saw away. It best NOT to use the saw Plate you are fitting but to use a saw with a thinner plate. The shim should be half the thickness of the handle minus half the thickness of the kerf of the saw you are using to cut the slot. Flip the handle over to make sure you are centered.
If you use the saw itself the set in the teeth will make the slot too wide unless the saw has no set.
If the saw has minimal set you might get way with it. Try using a carcass or dovetail saw for a handsaw plate, for instance.
There is a pretty good tutorial on handle making here:
It doesn’t have the guide shim trick, though.