Check the broken part
Before making the replacement you need to check the broken part in case the break was caused by the design of the stair such as the tread being too thin.
Create the new part
Measuring the gap and using the broken part as a guide you will need to make a replacement part the exact same size.
Fit the new part
Fit the new part and make sure the deck stairs are fully serviceable by just going up and down the stairs a few times making sure the stair structure and deck are working perfectly.
The stringers may also be the part that breaks first but replacing these can be just as simple.
Look at the damage to see what you can do about it. Because it supports the stairs its not so easy to remove while you make the new one. However the stringer can be measured accurately enough while still in the stairs.
Make sure the new stringer is addas the old one is removed to continuously support the stairs.