Four points

1. how to add noise: $q(\boldsymbol{x}t|\boldsymbol{x}{t-1})$ and $q(\boldsymbol{x}t|\boldsymbol{x}{0})$
2. how to remove noise (sample): $p_\theta(x_{t-1}|x_t)$
3. advantages

Points 1 and 2 are necessary for a engineering program. Point 3 is extremely important for doing researching