I replaced exhoust system on my 02 windstar van. Cadilac converters are catchin on fire ..why and what can i do

Not sure what you mean by "catching on fire" but it doesn't sound good- are there really flames?! if the Catalytic Converter is overheating, it could be due to back pressure somewhere in the exhaust system, or if things around it are smoking, try putting heat shields around it- they will get very hot, normally-

If the Cats are glowing red, there's too much fuel being dumped into the system. Don't drive it! Seriously - have it checked out. You will burn down your vehicle!