Starting at the causal effect, begin tracing around the exposure variable in a counter-clockwise motion. When you encounter a backdoor path, represented as a path from an antecedent into the exposure variable, begin tracing the directed edge to the antecedent. At the antecedent, repeat this counter-clockwise tracing process, and follow the backdoor path through to the dependent variable. Paths that consist of colliders and those that trace back to the exposure are not backdoor paths and will be skipped over when you encounter them during the counter-clockwise tracing.
When you reach the DV, record this path in the inventory, and then, importantly, backtrack to the prior variable. This backtracking preserves the history of the paths that have already been traced through the counterclockwise motion around each variable. Continue around that variable in a counterclockwise motion, and repeat the tracing process until all backdoor paths involving this variable are exhausted. Eventually, through the successive application of this counterclockwise motion and tracing, you will return to the exposure, and then repeat this process if there are additional directed edges pointing into the exposure.
When complete, you should have a numbered inventory of variables that start from the exposure and end in the dependent variable.
This process is summarized in a video demonstration.