Hi @GLH ,
So - for example - your current SharePoint list (created from Excel) sheet might say:
| Name | Newsletter | Email | Phone |
| Rhia | X | X | |
Is that right? How many rows are in this list?

If so, here is how I would go about it:
⭐Step 1 - Create your "Marketing Materials" column as multi-select, with the same names as above. So for example, Newsletter, Email, and Phone as choices.
⭐Step 2 - Create a Flow set on Manual Input and just leave it blank. Add a "Get Items" action, and ... get all items from this list 🙂
⭐Step 3 - Initialize a variable for each media type, as well as an array for the multi-choice column. Then, set the variable for each with the dynamic content from your Get Items. This will turn it into an "Apply to each", which is what we want.

⭐Step 4 - In conditions, say, "if this is equal to X, then, append this:
{
"Value": "Email"
}
to the array variable. You'd replace "Email" with whichever relevant condition you're doing.

⭐Step 5 - Finally, put in a SharePoint "Update Item" action with your array variable for your multi-choice column, and the ID from the "Get Items" dynamic content.

Success!
