How do you make Warhammer 40000 models with modelling putty?

Usually you don't "make" a model. I use green stuff from GW to add things like capes, nurgles rot, etc. Most of the time it's used to created extra details to personalize the models. From my experiences I wouldn't use it to put a model together, not only does it not hold well, its more expensive. I'm a fan of a glue called Zap-a-gap it holds well, but unlike plastic glue it doesn't melt the model together and you can take the model apart to reuse its bits. Hope that helps