The chair inserts are designed for toddler feed and play tables let the one below.
Step 1: Gather materials
If you intend to make a leaf for you table, first make sure you table opens up wide enough. Our table came with leaves that are less than 9" wide. The seats themselves have an overall width of 14", so you will need at minimum 28" and more reasonably 30" to do this. Alternatively, you can add one seat to each side, in which case you will just need 15-16" (to leave some surface outside the seat).
The table we have isn't ours, so I couldn't cut it. I also had to add new extension slides as the table originally didn't open up to the 32" overall I used. This also wasn't a cheap way to go. The pair cost me $70, but I had already ordered the seats and the laminate sheet, so I was in it until the end.
The seats were bought online through a link in the original post. They come in a set of six, which means I have four seats I need to sell ($20 each, send me a message). I remember it breaks down to $16 a chair before shipping. Originally, they were super discounted, but when I got there, it cost me ~$100.
You will also need 3/4" plywood, some dowel pins to match you existing table, and some scrap wood to widen the table leaf to the correct thickness along the edges (and over the extension slides).
You will also need contact cement, wood glue, screws or staples, a drill, a jig saw and a circular saw/table saw. A self centering dowel jig is also a great idea.
If you are counting, material cost assuming you have to go all the way is ~$250 if you have to buy a set of six seats and don't buy mine instead. If you have a leaf to sacrifice, your table extends far enough and buy my extra seats, it drops to ~$50. The high chairs we were looking at were $110 each, so I will be over. But, when they start eating PRIMARILY solid food, I will be able to cut the leaf down to 16", and make an insert in the middle to hold supplies and have it double as an activity table too.
So yes, I still lost, but the grand vision was there. That grand vision was two less things to buy and trip over. It is definitely worth doing in some cases.