Most people simply attach a tee to the air inlet of the engine’s carburetor. There will be a choke valve on the air inlet side that adjusts the fuel mixture. The other opening of the tee is the fuel inlet and the third goes to the carburetor. The advantage with this method is that the engine can be dual fueled with gasoline.
I have made my own carburetor but that is time consuming.
For a nice clean setup and no need to buy a mixture valve, because the original carburetor choke valve is used, is to modify the original carburetor. See the below video and photo. The fuel inlet is a reused vitamin bottle. This is were the fuel line is removed and the draft fan is used to start the gas producer.