Cut them into cubes that are roughly even in size.
Place the cubes in a a plastic bag that you can vacuum seal or use a gallon size plastic bag and try to get as much air out as you can.
Place into a 185 degree water bath for 1 hour. If they float, try to keep them under water using a small pot, just be sure the water level doesn't go above the max fill line for your machine.
Once finished, carefully remove from the water bath using a pair of tongs. Allow to cool for a few minutes before opening.
Ready to eat as is. Season with some fresh herbs. Or can be saved to be used with another recipe calling for cooked sweet potatoes.
Notes
How to make a bread crumb topped sweet potatoes:
Mash up the finished sweet potatoes
Spread onto an oven safe pan
Sprinkle a layer of bread crumbs on top
Place in 450 degree oven until the bread crumbs are toasted.