This can be really tricky, but this should shed some light on this topic. Before you start, CREATE A BACK-UP OF BOTH THEMES
Why are child themes created? The number one answer is for customization of a parent theme. So this should be your starting point when combining a parent and child theme. It should be noted that it would be easier to copy from the child theme to the parent. Also, delete anything from the child theme as soon as you are done with it (copied to or modified in the parent theme) to keep confusion out of the equation.
I should first start by searching and deleting/adding/modifying all the styles from the parent’s style.css that is in the child theme’s style.css. It is time consuming, but the worthwhile. If you start and work your way through methodically, you should not miss any styles. This part might be the biggest headache
Secondly should be templates, and also the easiest. Templates in child themes are modified templates from the parent theme, so these can simply be just copied and pasted to the parent theme. No special needs here.
Lastly would be function files, usually this would just be functions.php. You should work your way through and find all instances of remove in the child functions.php, and from there manually remove the function/action/hook from the parent functions.php. You should do the same for any function that modifies a parent function, but here you need to replace the parent function with the child function. All other new functions can simply just be copied and pasted
Care should also be take when working with functions.php. Most good parent themes’ functions are wrapped in an if ( ! function_exists( 'name_of_function' ) ) statement. In the child theme, these functions wrapped in the conditional can simply just be overwritten in child themes. These functions in the parent can simply just be replaced with the child.
This should give you some kind of guideline to get started