I have made the model of the wings, including wings, fuselages, vertical/horizontal stablizer,
but I did not follow the instruction (RWDC_how.pdf) since it was really outdated.
But I did used the correct airfoil for the wings. Does this mean I have to restart the whole model?
If I can use my own created model, what is some factors I have to measure to make sure it works?
You do not have to restart the whole model if you just updated the wing. All the models are modular. If you only updated the wing, you can keep everything else the same. In the assembly file, make sure you attach the updated wing to the fuselage. It should fit as long as you kept the chord length the same.
You are strongly encouraged to create your own CREO models from scratch. As long as you show the wing, fuselage, empennage, landing gear and payload, you should be all set. For sizing, make sure your wing is large enough to generate enough lift to be able to carry the payload you have selected.
So does that mean If I am creating my own airplane, lift is the only measurement I need to measure, or is there any other factors? Also, is it mandatory to use the worksheets (mathcad) to create the model?
Nothing is mandatory in this competition. You are welcome to use the tools you are provided or design the whole thing without using any of the tools. Similarly you are not required to measure lift - but if you want you can do that and perform the entire aerodynamic, sizing and performance analyses.