I am finally having some success intentionally growing ferns from spores. I collected and sowed spores from some interrupted ferns (Osmunda claytoniana) over the summer. They have been hanging out as gametophytes for months now and some are finally starting to grow sporophytes. Here is how it worked for me:
I kept my eye on a batch of adult plants this summer. Once their fertile fronds developed I would flick them every now and then to see if they were releasing spores. Once I saw that they were I shook the fronds over some paper to collect the spores. I then took some old potting soil and sterilized it with boiling distilled water. I use old takeout containers because they are small and have clear lids that form a seal which keeps the humidity high.
Once the soil was cool I sprinkled the spores over it and then placed it on a shelf where it gets a small amount of ambient light every day. The rest they did themselves. You just have to remember to check on them and keep the humidity quite high because they can dry out really fast. They seemed stuck as gametophytes for months. I just noticed the start of these sporophytes the other day.