The day before: cut off the bone or cartilage. Trim off the sides for your block (so that it's ready to be sliced).
Wrap the trimmed block in plastic wrap and freeze.
The next morning: It'll be frozen hard! Transfer to the refrigerator, and defrost for 2 to 3 hours.
2 - 3 hours later, it'll be semi-defrosted and icy, and easy to cut. Slice thinly as you like!
The meat is still ice cold, so cover the meat with the plastic and a towel while cutting.
Look! Look! It's this thin! Finished!
Also see [Recipe from Overseas] Make at Home! Butterflied Chicken Breast (Recipe ID: 1371504).
Story Behind this Recipe
Thinly sliced available at Japanese supermarkets is not sold overseas. If you order from the butcher directly it you can get it, but since there is no custom of eating thinly sliced meat overseas, it can be tricky.
When trimming the meat block in step 1, don't divide into smaller chunks. Freeze the meat in a single block as it'll be easier to slice. You need to put your weight when slicing the half-frozen meat, so it's pretty strenuous. If you don't have a lot of strength, try defrosting the meat for 3 hours! Or, use a serrated blade knife.