Hi Carlos
First you can start with create hypo-componets for your G(CO2) and G(CO). This requires the actual MW, boiling point and density. After finishing hypo-component generation, you will have 2 new components for using in your reaction.
Next, you can go to the reaction section. In ASPEN (not sure which Aspen you use: hysys or aspen plus), there are many reactor types e.g
RSTOIC, RYIELD, RBATCH, RCSTR, etc which require different input as well as the accuracy of the output. You have to choose what input do you have and then matching with the reactor in the program. The difficulty of this step depend on your reaction complexity. If your reaction has a very complicated machanism, you need to select the rigurous reactor model e.g. RBATCH, RCSTR.
Hope this may help you