With your hand. You have to really watch their body laungage though so things don't get to rough. Like my girls I have to be a bit softer, they get offended easily. My boys, well I've chased them around, pinned them, been chased and flipped, and they don't care. When I do it I make sure that I don't always win. Sometimes I just flip my hand and let them pile on as if they pinned me. Then I go back to bouncing around and tickling their bellies and they go bouncing around me.
I can never tell if my rat is enjoying it! She doesn't get mad or nip and she always runs back for more, but I can't tell if she's wrestling back or just trying to get away from me! Hopefully she's more into it when she's fully grown...
I've found you just have to watch. Like if they bounce and popcorn away and back again, that's usually a sign they are having fun. If they run and eye you, it could be more "I don't like this" or "I'm not sure". With the boys they always act like they don't like it, like they want to get away, but then they run around in a big circle and end up on top of my hand again. And they lick me a lot. My Cricket, I learned, disliked having me go after her rump area. I looked around and found that rats tend to aim for the rump when they are really fighting, while they go for the scruff when they are playing. So that's what I try and copy. It's hard to remember. XP I have a bunny who likes me to scritch his butt, and he binkies and bops around when I do. But the rats seem less enthused if I do.
Baby Gizzard LOVES it. She comes back for more. Chai doesn't care either way, Wafer and Oreo get too aggressive if you do it, and Mocha gets so insulted she ignores you the rest of the day if you do it. Each rat is different. They will let you know their own special way!
To add just a bit more to the topic, younger rats around 6 to 10 weeks old are especially fond of hand wrestling.
Our part wild rat was actually particularly aggressive during play and actually frightened us, my daughter then only 5 years old was nearly brought to tears when her rat 'attacked her' from out of the blue. I put that rat on my desk to try and understand what was going on and she leapt from my desk right into my face and in a matter of a split second had mock bitten my lips and nose several times... I was actually rather shocked and swatted her off, no sooner did she hit the ground did she bounce back and charge up my leg to carry on her ferocious attack snapping at my finger and any exposed skin she could find.... As I was caught so off guard and I really didn't want to hurt her, I was swatting and doing my best to get a hold of her.... Then I suddenly realized I wasn't bleeding and I wasn't in pain. Neither my face nor my hands were in any way punctured or otherwise injured and this bazaar insanity was only a game. Forget about hand wrestling... this rat went right for the face, nose, eyes and mouth... and she didn't bite once, she literally chewed at me... but she didn't break even my most sensitive skin. And a two foot jump from my desk and a 4 foot drop to the floor, were all apparently fair play. I explained to my daughter it was only a crazy game and her rat wasn't actually attacking her and for a couple of weeks it was pretty crazy around the house... with unexpected sudden attacks being launched out of the blue by our little wild child. My daughter especially enjoyed the crazy chases around the house, and I have to admit so did I. I'll also add that my instincts were always to fight through until I won and I advised my daughter to do the same... I'm not going to say that there was any good reasoning behind this, but something just felt wrong about letting a rat that can do so much damage so fast get the upper hand. Usually we'd fight for quite a while before I'd snatch her and give her a big hug to calm her down....
Just as quickly as it started, it was over within a couple to a few weeks and she never really play fought again... She lived outdoors on her own for 5 months, and she tore up my neighbor's hand when he tried to grab her, and she punched a couple holes in Fuzzy Rat's trachea when Fuzzy Rat tried to dominate her and she took out a chunk of my palm when I blocked her from killing a mouse and if she got upset she would leap into a ball of fur, fangs and claws... She was actually a very sweet rat, the kind that would nap on your lap, but if anyone or anything upset her she was capable of doing real damage fast. Those play fighting skills became survival skills and made her a rather dangerous little friend to have. Oddly, she never got upset with my daughter who would routinely squish her into doll outfits and play with her like a rag doll. I handled her with care never to upset her.
All of the domestic rats we've lived with have play fought too, but mostly they only ever attacked our hands and their attacks were more hit and run than full on no backing down brawls. But I'm pretty much convinced that the fight games domestic rats play are left over from their wild ancestors when mock combat was practice for establishing status and developing survival skills.
I always look forward to the play fighting stage when we adopt a new rat pup and I'm always sad when it's over. Once rats get older and more serious, they usually stop. Our big girl Max is a very serious rat, with little sense of play and gets annoyed with foolishness easily. Cloud, now about 10 months old can still be coaxed into a little tussle every now and then by my daughter, but overall she's a grown rat too with serious rat business to attend to... If I had to make a final comment, I'd say the more self important a rat feels, the less likely he or she is to play fight or play at all. Rats that think of themselves as high status seem to look at life more seriously (like life is their job)... Rats that aren't all about being in charge tend to stay more playful throughout their lives. And just maybe, the same holds true of humans.
For the most part, enjoy play fighting as long as you can, when it's over you'll miss it.