Go to the debate. Write down the things the debaters say. Evaluate those things as honestly as you can, based on how persuasive they were to an unbiased intelligent observer. You then allocate speaker marks and positions accordingly. For the first six of the nine preliminary rounds, you give teams their positions (not speaker marks) and feedback. For the latter three rounds, you tell the teams you saw nothing at all until after the break has been announced.
The mark bands below are rough and general descriptions; speeches needn’t have every feature described to fit in a particular band: our job as judges is to find the best fit. Throughout this scale, ‘arguments’ refers both to constructive material and responses.