The basic expansion of parallel processing can be achieved by an evolution of just one simple technique: Counting to 100.
At the most basic level, all you have to do is have your tulpa count from 1-100 without any help from yourself, restarting when you notice an error or if they stop.
Repeat this process until they can achieve it consistently and without error.
Now, once you feel proficient at this, start increasing how distracted you are from your tulpa's counting, only barely listening in to check accuracy when possible.
My recommended progression is:
-Listening to music -Watching TV/Youtube/etc -Playing a video game -Talking to another real-ass human being
You can also increase the number to higher than 100, as well.
You can also additionally compound this with your visualization process to improve consistency of visualization over time, which is particularly useful for imposition.
You just adjust things slightly by instead having your tulpa count on their fingers(or equivalent), or drawing/painting out numbers, either in reality or in wonderland. Feel free to augment this challenge in the same ways that you can with the voice-only method.