look at purity of the item you bought on the site or by documentation if provided. a gas purity of 99% means that if cylinder is hooked to a oxygen mask and all inside volume of the mask is replaced by the gas so inside concentration of the gas equals 1,000,000ppm or 100% of the mask volume. so you can possibly get CO concentration from range of 100,000 to 1,000,000ppm depending on a gas mask isolation (a leaky mask leaks the gas into atmosphere)
CO cylinder can be easily connected to oxygen mask via silicone tubing of proper diameter. set up the gas flow rate on pressure regulator to 15L/M for 7 minutes of gas flow, it's time until the cylinder contents are exhausted or on 10L per minute as overall gas concentration above 10,000ppm(1%) is deadly itself and you can ignore possible gas leaks at such purity and speed of flow.
also make sure you bough CO and not CO2, i never heard that CO used in welding though