You have to make a small cutout in the stock along the LH side of the receiver. You must then drill and tap two holes into the receiver wall, assuming your rifle isn't one of the somewhat uncommon "ex-sniper" examples with the two holes already there, in which case you can just clean out the holes and bolt the mount on.
A bent-handle bolt body is required because the straight bolt handle won't clear the scope. An original Soviet bent bolt handle won't clear the factory stock, so you will have to cut a groove into the stock to accomodate the handle. OTOH you can buy aftermarket bent-handle bolt bodies that clear the stock and eliminate the need for the groove.
BTW if you didn't know this already, the bolt body, which includes the handle, doesn't affect the headspace of a Mosin-Nagant. It can normally be swapped freely with no negative consequences. For this reason, if your rifle's numbers match, I would recommend buying a second bolt body for the bent-handle swap rather than hacking or welding the original one. This way you can put the rifle back in its original configuration in seconds.
"Smokey, this is not 'Nam. This is bowling. There are rules... MARK IT ZERO!!" - Walter Sobchak