there are companies that will come out to cut the foundation with a saw. It is a bit messy with water to cool the blade, but other than that, your ready to frame in the window.
Use pressure treated against the concrete and make sure the bottom of the window is 44 inches off the floor minimum when you measure it out.
Start digin and make sure you meet the local codes for exit stairs....some towns want more risers, some just follow the code they have adopted.
Here in CT we are still on the 2003 Irc with additions to the code....only 7 years behind.