You "simply" add enough two-part polyurethane foam to displace the weight of water about equal to the weight of the boat. Why 'about'?
The boat itself displaces a significant amount of water. We can help you determine the proper amount of foam.
For doing some simple calculations, consider that seawater weighs approximately 64 lbs. per cubic foot.
Installing foam flotation is not without risk. There are many, many, stories of boat repair shops removing great quantities of waterlogged foam. Be very careful in your foam selection. Do your homework.