Before you start to fill up the cartridge with the DI resin, insert one of the sponges, preferably the thicker one, fully into the cartridge.
Side View of the sponge
After that you can use a spoon to scoop the DI resin from the zip lock bag and place them into the cartridge. Put the resin into the cartridge until it is full like this.
After that, place the remaining sponge on resin and screw the base cap on the cartridge. What you will finally have will be a DI cartridge that is fully packed with DI resin.