Finding "level" is probably the most important part of building your rink.  You MUST have 3" of ice to hold an average adult, so you need to ensure that the "shallow" area has at least 3" of water.

 This can be done with a line level, laser level or water level.  Personally I use a laser level.

1. Find the area that you feel is the "high" point in your rink and set and level a block of wood, so that the line of the laser is 3 1/2" above grade.
2. Put stakes at the corners of your proposed rink and every 4 feet between the corners
3. Point the laser at each stake and mark the point that the laser hits it
4. Measure from the ground to that mark
1. You now know the MINIMUM height of the board at that point
1. If you used just that board height your ice would be flush with the board
2. I would add 3" to that so you have boards above the ice surface to keep the puck IN THE RINK
2. Do this at each stake
5. Purchase the appropriate 2x? to create your frame
1. I cut the bottom of my 2x(s) to go with the slope of my yard so that the top of the frame is level, but you do not have to do this