For a temporary fix you'll need 4 things:
1) grease gun
2) marine grease (or any bearing grease in a pinch)
3) 1/8 NPT grease nipple - all I had to start was a 3/8 nipple but I couldn't get a sufficient seal to get the grease into the hub
4) Generic 1.98" bearing dust cap - optionally a plumbing PVC collar - 2" - 1-1/2" slip collar - which can be used to force the cap on. I didn't have one and deformed the cap putting it on, but the caps are fairly cheap to get.
This image shows the 3/8 nipple that I initially tried. I could only get a bit of grease in with it and it took a lot of pressure to get the grease moving. But it was sufficient to push some water out and get us to an autoparts store where we could get another nipple. At the parts store I asked for a 1/8 NPT nipple, but what the couter guy said was 1/8 was way to big so I ended up using a 6 mm nipple.
I didn't get a picture of the bearings filled with new grease. However, I used a small plastic bag to capture any excess and I pumped until I had new grease pushing out it in a complete circle. Initially I had to really push on the grease gun to get the grease moving. They use a fairly generic spindle so the grease is pushed through the middle to the inner bearing and then flows back out through the outer bearing. Ideally I would have jacked up the trailer and spun the wheel to ensure that the new grease was evenly distributed.
Bad picture, but the cap is deformed as I just used a piece of 2x4 and a hammer to fit it on.