I divorced in 22, sold stock and mutual funds that were jointly owned by my ex and me. How do I make him a nominee so we can split reporting the cap gains and div?

Allocating the dividends is relatively easy and I will give you the steps below to show you how.  The 1099-B is a separate and more difficult situation.

You can split the income that was reported on the 1099-DIV by taking the portion that applies to the others and listing the income as "nominee" income assigned to you but belonging to another.  These are the steps you should follow for the assignment of the interest income:

1. In TurboTax, go to "Wages & Income"
2. Scroll to "Investments and Savings"
3. Click "Edit/Add."  for the particular dividends to which this applies.  If not initially entered then enter the information as shown on Form 1099-DIV.
4. The next screen will ask if any uncommon situations apply, checkmark "I need to adjust" and click "I need to adjust these dividends".
5. Place the amount of the dividends that applies to the other in the adjustment amount line and click "you received all or part for someone else".
6. Click continue and the portion of the dividends that applies to the others should be deducted from your income

The 1099-B from the Mutual Fund needs additional steps.  As you enter the data, and if the 1099-B is totally in your name, you have to adjust your cost basis for each sale while noting that you are acknowledging 50% (my assumption) of the proceeds and such.  The steps that follow should assist you:

1. Click on Wages & Income and scroll to Investments
2. Click on Add investments
3. The mutual fund 1099-B may break the separate types of sales, such as long-term, short-term, etc., down separately but with totals for each.  If this is the case you will have to enter each "group", such as short-term covered, long-term covered, as separate transactions.  The entry screen initially will look like the following (the numbers are for example only):  

The next section of entry will appear like the following:

Click continue and then none of these apply.

The next screen is your adjustment screen for your basis.  Enter the number that will only net to what is your income for that section.

Click continue and you will see a summary of the transaction you have entered.  In my example, the gain should only be what is your portion and will be reported that way on your 1040.

Repeat for each section of the 1099-B and you should be fine with what is yours and what is not.

@yyslaine