Plain finely crushed flake fish food that you feed the adults will be fine for guppy fry food...they are not picky and will eat most anything given...they are fully formed and ready to eat within hours of being dropped.
Some female guppies will eat the fry as fast as they are dropped and other will not eat any or just a few
By providing both upper level and lower level hiding places in the form of floating and rooted plants and moss- will help increase chances of more fry survive-either the mother or other fish eating them.
The more volume of water the better, I recommended a full tank to give the fry more space to get out of the mothers way and/or line of site to increase odds of more surviving.
Be careful over stressing the mother-this can cause her to abort or premature drops and the fry usually will not survive
Even without a male-she should have roughly 4-6 sperm packets stored and should drop fry every 28-31 days for the next 4-6 months.....
Livebearer fry are one of the easiest fry to tend to in my experience-easiest to feed and grow out...most tolerant of ranges of water temps but tend to do best in harder water.