When you log in to BBO, you probably see the following at the right side of your screen.
If you do not, then click on the Messages Tab at the far right of your screen.
Now click on the Account Tab at the far right.

This takes you to the Profile Screen.
You probably have already filled this out.
You can now change things (except your login) if you wish.
You can leave your name blank, use only your first name, or make up a fake name.
If you want to identify your skill level, click on the appropriate line. Or just leave it at private.
You should identify your country as USA. When looking for substitutes, Directors look for English-speaking players
If you make any changes, click on the Save Changes button close to the bottom of the screen.
If you do not need to make any changes (or after you have saved any changes),
then click on the Convention Cards tab at the Top of the screen.

After you click on Convention Cards, the Convention Card List screen appears as follows.
You can either click on New ACBL Convention Card (to create a convention card from scratch)
or click on an existing card (e.g. SAYC card) to start with that card, and then edit that card.

Below is the screen if you clicked on New ACBL Convention Card.
(If you click on a stock card, a screen will appear that gives you the option to view or edit that card.)
You fill it out just as you would expect except for two items.
Fill in your partner's BBO name in the Partner box at the top (just under the Save Changes button)
Fill in a descriptive title (using some combination of your name and partner's name) in the Title box;
this title will appear in the list of convention cards once you have saved the card.

If you want to use the same convention card with more than one partner, open the card for editing,
change the Partner name and Title, and then Save the card. Both convention cards should appear in the list of cards.
(Or see the procedure below if you use the IDENTICAL card with more than one partner.)

What if I am getting ready to play with a new partner and do not have time to create a card?
You can use an existing card.
Click on an existing card (either one of the BBO cards or one of your personal cards),
and then fill in your partner's BBO name to the right of the Use with Partner tab on the screen.
Then click on the Use with Partner tab.
See the screen below.
(This might be the solution for using the same card with more than one partner.
Create your card that you use with several partners,
but do not put in a partner's BBO name.
Just title the card something like Card with Tom, Dick, and Harry.
Then use the procedure below whenever you are playing with one of these partners.)