Things You'll Need
Plug the Thiele-Small parameters of your woofer into a ported (or vented) enclosure calculator. Woofers with a Qts of .4 or lower are suitable for vented or passive radiator designs.
Use the inside cabinet volume derived with the speaker volume calculator to design a cabinet suitable for vent tuning, which also physically accommodates a passive radiator or multiple radiators with roughly twice the moving area of the woofer.
Use an Internet port size calculator to find the inside cross-section area and length of the right vent for your speaker/cabinet.
Multiply the vent area and length to get the air volume in cubic inches. Divide that figure by 47 to get the air column's mass in grams. Your passive radiator's published moving-mass specification should center around the same mass as the column of air in your calculated vent size would weigh.
Tune the passive radiator to a lower frequency by using a threaded rod or bolt to add mass. This will make the system behave like a sealed system above the initial tuning frequency, but a vented system below that frequency, making the bass tighter and deeper. Tuning hardware is usually supplied with commercially available passive radiators.