The tile is the first step. After that it was time to plan the overlay to go into the foam. For this I used parchment paper - wax paper or tracing paper would also work. I used some railway track pins to pin the paper. You will also noticed I darkened the river - this was more for the purpose of the blog to highlight the area I was taking about.
Once the area (water only) was traced from the tile I then cut and transferred the parchment paper to the foam. Pins or nails are used to hold in place.
Its important that you leave the outermost line as your top level and bank in from there. I used a knife to cut through the paper. You can achieve the same result simply by retracing the outline and pressing hard on the pencil or pen.
Once I had the rough shape (literally) I cut away the excess materials. I used a series of "V" shaped cuts to work down to a depth - halfway into the foam. You really don't have to be too careful at this point as you can later use sandpaper to smooth the bottom. In a later stage you will see way this doesn't need to be perfect.
The final process for this particular phase is to paint the bottom white. This does two things, 1st: it helps to seal the loosened foam, 2nd: it helps to hide the yellow foam below the water surface.