As of today, I've had my guinea pigs for two months. I'm not sure how much advice I can give, because I'm still very much learning, but I found that covering my guys' cage with a blanket for the first week or so, gradually lifting the sides of the blanket each day, helped. It was basically like a giant hidey where they felt safe. They could hear and smell me, but since they were protected and the scary predator couldn't see them, they were happy to venture out and explore and get to know (and destroy) their cage, and were feeling comfortable by the time all sides were off.
Also, talk! Talk, talk, and talk! Predators don't give away their presence so talking helps to settle them. I sometimes just sit by the cage and chatter away, and they very soon learnt to ignore me. I also have funny little mantras, sayings, so they know what I'm doing or about to do by the tone of my voice. Things like "You're ok! I'm not interested! I'm not interested! Just getting a drink!" for when I stand up and move around and leave the room or "Just sweeping, just sweeping!" (my cage cleaning song, although now it includes "Merlin, get off the dustpan! Merlin! Merlin!"). It sounds and feels silly, but they do recognise it and settle down.
I also made sure that my interactions with them and their cage didn't just involve trying to pick them up, and, well, interact with them. Poo picking, I basically ignored them to begin with (now, it's a case of them being "OH NO YOU DON'T! YOU ARE GIVING US FOOD! PAY ATTENTION TO US, SLAVE!" and not letting me ignore them), so they get used to you being around the cage and it not meaning they're immediately going to picked up or stroked and that the scary creature isn't going to eat them.
Food is also very much the way to a piggy's heart. My two will literally climb on top of each other (Arthur literally does climb on top of Merlin's head) for pellets. They love them. I started off offering the food between my fingers, so they barely had to come near me, and when they were happy taking food from me that way, I'd offer it open palmed, so that they had to take the food from my hand, and then I slowly started touching and patting them, so now they're quite happy to let me stroke them because it means food! delicious food. I also made them come to me. They now line up at the edge of the cage the moment I rattle the food bag (they've finally started wheeking), and know they have to put their feet on the side before they can have some. If they don't come to me, well, no extra tasty pellets. (I do still feed them, promise.)
It's gradual little things that help to build up both your relationship with them, and their confidence. It definitely takes time, but every new little thing is so rewarding!
