We sure hope that custom alerting isn't complicated!
The best way to tackle custom alerting is to think about the 4 types of configuration needed for each custom alert:
I would recommend taking a look at the Dynatrace documentation for custom alerts and static thresholds:
Is there a particular part of the custom alerting configuration that you are having trouble with? They should generally be able to be configured very quickly if you have an idea of what metric and entities you want to alert on.
Although my examples were of frontend apps, the information applies for other entities including services. As for filtering on multidimensions, using the option Add rule-based filter provides the ability to filter on any of the dimensions available for the metric. If the option that you are looking for for that metric doesn't exist, you may be looking at the wrong metric.
The metric selector will show all of the metrics available. To combat the list being too long, the Category option exists to better filter what you are looking for. In addition to scrolling the metric list to find your metric, you can also type in the field to quickly filter for text patterns. If you are having trouble identifying which metrics you want to use, I would suggest using the Metric browser to get familiar with different metrics and their details including supported dimensions. It can be found by using the navigation menu on the left of your screen, under the category Observe and explore. The option is called Metrics.