Hey Night. Well as for offerings for the Goddess, if you are going with food she likes the following: organic milk, mead, pastry, white wine, riesling, apples.
Here are some herbs that can be offered to her: Lavander/Rose
I also suggest you doing some research yourself to see what she likes. As for giving an offering if you have an alter made for Frigg you may place your offering there. Most offerings should be placed outside eventually if you are able to. It is the energy they take with them not the physical object itself. If you decide to give them a rock/crystal or statue it should be kept on their alter if you had made it for them, if not just a designated spot. If it's something that could go bad I suggest leaving it on their alter for a day then take it outside or place it in the compost. If you are giving them food, I suggest do not eat it, you gave it to them personally, you may give it back to the earth when you are done.
Frigg is a goddess of domestication in my opinion and experience. So, cleaning of your home, taking care of family, cooking dinner and checking in on the sick and elderly can all be used as acts devote to her.
Frigg likes hand made crafts. Quilting, crochet, needle point, embroidery, spinning, weaving and knitting are all activities she is interested in. If you make lots of stuff and don't have a use for it, donating to charity would be a good devotional act.
Things like gardening (either flowers or food), tending animals (rescue and fostering), and cleaning up trash around the neighbourhood are good out door activities.
For food and drink she isn't really all that picky. I find she likes wine and mead. She enjoys breads, cookies, cakes and muffins.