Some of the steps I have outlined in a previous post on this thread. Minnesota has a great resource in their MNGEO access portal for spatial data. Minnesota has committed to open data community as far as making it available to the public. Not all states and counties subscribe to that concept and will charge for data as a way to make money or at least recoup their cost of creating the data. Your mileage will vary...
I will limit my comments to electronic spatial data and assume that you already know how to import an image and convert into vectors. I have converted both scanned and electronic pdf's of printed images. Huge pain but it can be done and sometimes the only option. My Masters degree is in GIS Geographic Information Science. Essentially it is working with electronic spatial data. It takes a special type of software and more than a little bit of training to work with. I have two packages I use, ESRI's ArcMap and QGIS which is an open source package. ArcMap is super expensive and I wouldn't have a copy except for I have it at work. QGIS is free but not as powerful and somewhat limiting on what you can do. But it's free and I have it installed on my home computers.
First I find my bathymetric data and download it. Usually I will go with ESRI shapefile formats since that is the most common file format for spatial data. I also work with DEM's (Digital Elevation Models) which is a raster with depth as the cell value. The shape file is loaded into ArcMap and the lake and all of it's contour lines are selected and then exported as a dxf. The dxf is then loaded into VCP using the File Import Import Vectors menu item. Depending on how you set up your project you might not see anything happen. That is because VCP project is in inches and the dxf is in a different unit. Remember you are importing something into a small project (12x12" maybe) with a size that is measured in miles. It will be huge. I do a select all (cntl A) and then use the size button and reduce the size to something I can see in my project.
Once your have the size right for your project it is time to start setting up your toolpaths. depending on the depth of my wood and how many contours I want to show drives the size of the pockets. Let's assume that I have a full 1" piece of wood, my lake is 60 feet maximum depth, each bathymetric line represents 10 feet, so I would have 7 contours lines including the shoreline. I can make each contour .125" deep and still have 1/8th of an inch left on the bottom of the lake in the wood. I select the shoreline and using a pocketing toolpath pocket to a depth of .125". I then select the next contour line which would be 10 feet depth and do a pocket of .125 and have a starting depth of .125. When that is done I select the 20 foot contour and do a pocket toolpath of .125 and a starting depth of .25 and so on until I reached the bottom of the lake. It can get confusing when you have a lake with a lot of variation of depths. You may have to select a bunch of contour lines when you have lots of holes and flats in a lake. Just make sure to select all the contours that represent that same depth when doing your toolpaths.
My humble opinion given the number of bays and the size of Leech you would be biting off a lot for a first project. It's a very large lake, lots of small detail, irregular contours, and lots of structure. Given that much of the lake is shallow with a couple of deep holes much of the map would be pretty flat with little depth. I have done a number of projects for Lake Minnetonka residents and in the end just make it for the particular bay or arm they are in. Unless your project is going to very large and you have the capacity to do a very large map I think the results will be too much detail for the size of project.