Once you’ve had a bowel motion/movement, remove your colostomy bag as you would normally do so when changing it. Using the instructions provided with the test kit, the sample should be taken via the pre-cut hole in the bag and not directly from the stoma. Do not cut the bag open as this would not be necessary.
Best practice would be to take the sample as soon as you’ve had a bowel motion/movement, however if this is not possible, you can take the sample as soon as you’ve changed the bag.