Crawling down stairs can often be hard to teach. Why? Because when you are teaching going up stairs you can place a toy in front and they are motivated to try to get that toy. Going down stairs, if you put a toy at the bottom, they will often want to turn to look at the toy and then try to go down head first instead of feet first!
How do we recommend working on this? We recommend starting on the couch and bed and any other surface that is off the ground early on. Pick a consistent phrase like ‘feet first’ or ‘on your belly’ and help them get into the position and then guide them down. As you practice this start to let them initiate to see what they will do and then correct if/when you notice they are trying to go a different way. This way they get a chance to practice trial and error. Stay close by in case they lose their balance or move too quick (if they do this, put them back up and take them through it the correct way). We need to give them a chance to make adjustments based on the feedback their bodies are getting.
As they get better at this you can start with 1-2 steps going down. Use the same cues and you can even just take them completely through the motion. Sometimes I have them slide down on their belly with me pulling at their legs. This lets them see that they can get down that way and what they want is at the end. Also use the same consistent phrase. As they get more comfortable bring them up more stairs and try again.
Repetition and consistency are the keys to mastering skills!