One more time...
It's Foley with a capital F.
Also, you are not doing Foley if you are not performing live to picture; you are collecting sound effects if you are cutting them in after the fact.
I really recommend against going out into the field to record footsteps; unless you close mic (I mean REALLY close) you will pick up too much extraneous noise.
You should be doing all the Foley work on a Foley stage, or a reasonable facsimile thereof. I don't have the room for Foley pits so I had to improvise. For dirt, grass, leaves, etc. I put a blanket on the (carpeted) floor of the studio, put down (clean, unused) cat litter and select the appropriate foliage noise makers. I have shredded cardboard, various "Easter" grasses, old recording tape in various widths, shredded newspaper, and, if the season is right, I'll use real leaves. I have a bucket of aquarium pebbles for gravel. I mix and match all of these items until I get the sound that I like. I have a number of 3' X 4' pieces of various types of wood for hard floors (oak, light and heavy plywoods, etc.); some of them have vinyl and hard tiles glued to them. If I want a more resonant sound I put them on a platform that I built. For concrete I use the basement floor just outside my studio door; I put up a couple of Gobos to deaden the basement resonance. For water Foley I put down a tarp and use a kiddy pool; the wet vac is handy for accidents and to drain the kiddy pool after the fact. I then do mic selection. As my studio is small I have to close mic; otherwise I pick up too much "room."
As for levels try between -30dB and -20dB. Keep in mind that it is not an exact science. The noisier your mic/recorder the hotter a signal you want; otherwise you end up piling noise on top of noise.