Try using visual cues, or transition cards

Some children on the spectrum find visual evidence, such as transition cards, to be more effective in easing their troubles with transition. Using a consistent visual cue to indicate when it is time to transition can help to avoid any confusion for your child and to allow them to develop familiar transition routines. For instance, giving them a visual cue, either a photo of them or any type of visual symbol that indicates “go check your schedule” can help them to match the cue with the designated location of their schedule. Upon viewing the schedule, he or she will then be able to predict the transition routine. 

A "finished" box could help make transitions smoother

A “finished” box is a box where your child can place items that they are finished with when it is time to transition, or things that represent those items. This box can be located anywhere that you perceive to be suitable and it can be labeled with a word or a visual cue to indicate its purpose. The drive behind introducing a “finished” box is to create clear and predictable transition routine, all while decreasing transition time and heightening positive behavior.

For example, you could put literal things in the box, like a homework assignment, or you could write out "Homework assignment" on a piece of paper and have your child put the paper into the box, to signify that it is finished.

Supporting your child during transitions is essential

Factors such as the length and difficulty of a particular task or activity, the specific needs and strengths of your child, and their interest level may each contribute to trouble with transitions. Similarly, if a setting is too crowded or noisy, your child may resist transition to that particular location due to their sensory perceptions.

The previously mentioned transition strategies are just a few approaches to sequencing certain activities with an attempt to build consistency and to reinforce positive behavior. Keep in mind that it takes time and practice before you can see improvement and different transition strategies may be appropriate in different settings. By continuously enforcing the use of these strategies, your child will be able to move more easily from one task, activity, or setting to another, increase their independence, and more effectively participate in activities at home, at school, and in the workplace.

References

https://childmind.org/article/why-do-kids-have-trouble-with-transitions/

https://chicagotherapeuticpreschool.com/why-are-transitions-difficult-for-children-with-autism/

https://iidc.indiana.edu/pages/transition-time-helping-individuals-on-the-autism-spectrum-move-successfully-from-one-activity-to-another