Per your request, Sandy, I am posting how we did the modification for an apron front sink. Bear in mind, these are IKEA cabinets that are very "hacker" friendly. The base cabinet is 30" and the sink is just shy of 30"; it's almost 20" from front to back and 10" deep.

I didn't take photos of the entire process, but I did take a few. Hopefully, they will better illustrate my attempt at explaining how we did this.

First we turned the sink over and measured the sink, itself, (three times for accuracy.) and using a straight edge, drew the outline on the base cabinet sides, with the cutout to the front of the cabinets. We basically notched out the sides of the cabinets where the sink would sink, keeping in mind that the top of the sink had to be flush with the top of the cabinet sides. Since we want our sink to sit somewhat proud of the cabinet, we made our cut to be 19" deep by 10 1/8" high. Hubby used the table saw to make the cuts.

I assembled the cabinets, as I normally would, just without the front brace, as that is where the sink sits. We carefully turned the cabinet over the top of sink to make sure that the top of the cab sides would sit flush with top edges of the sink. We then placed the rails over the sink bottom, so we could make certain the disposer would fit within these rails, and that the rails would properly cradle the weight of the sink.

We did this by actually turning the disposer upside down and placing it over the sink hole. We then ripped 1" from the 2X4s. We clamped the rails on the sides of the cabinet and measured then cut our cross braces. We removed the cabinet from the sink before DH pre-drilled holes on the sides and then screwed the rails into the cabinet sides.

We turned the cabinet upright and placed the sink on the supports to see the fit. This is one picture I forgot to snap. :^/. Sorry.

Here is a top view of our sink supports:

We again removed the sink, and I finished the cabinet by nailing the back onto it.

I will take another photo later so you can see the sink sitting on it. DH is at work, and it is just too heavy for me alone to get it on there.

I hope that helped a little.