I'd say go in after a hard rain, find the soon-to-be-ponds (pooling
water) in the landscape and dig out the ponds when it's dry
enough to dig. You can also find the ponds by finding contour lines using an a-frame or two stakes and a tube of water.
That's what I'm doing. With the dirt you dig out of your new
pond, you could make a hugelkulture on the low side. 2 awesome elements with one dig.
I would also suggest doing it in increments. That way you can look at it for hours, not minutes, and make little adjustments here and there. It takes more time, but you won't make any huge mistakes. Plus you can modify where the water stands by digging more here or there.
The point with heavily watered
land in the winter is directing water to the places where it's okay to pool in the fall-winter-early spring and having a water resource for summer. When you slow the water down, it's also recharging the water table, so in the summer the plants won't have to dig so deep for a drink.
I also bought a small pump to pump the pond out and put the water higher in the landscape (a second pond uphill).
I don't have any water access on the land and every time it rains heavily the pond gathers about 1000-2000 liters. I run half uphill and half goes into summer holding tanks.
I used to have a huge problem with flooding on the downhill side of the land. Now my only problems are a little area that gets too wet and we walk on it (will build up and sow), and a future mosquito problem in a place that already gets a lot of them.
William