start with a regular artist insert or update page as created by the data assist wizard.
In the Insert or Update record behavior, not the Store As name in the session variable section, make sure this session exists on the bindings panel, if not you will need to add it manually. Click the plus button and select session variable and set the name to the one from the insert or update Store As.
Then create a recordset for the instruments.
Add a checkbox element to the artist form for the instruments, set it's checked value to be the instrument ID, and add a repeat region around it to display a checkbox for each posible instrument.
Then add the Manage relational data behavior.
On the first step of the wizard, set the trigger and select the relation table (This is the new table you have created)
On the second page, you set the main table join. In this case, the column that joins to the artist table. for the foreign key select the artistID column, set the type to number, click the lightning bolt next to value and select the Session Value from the Store As section of the Insert or Update behavior.
On the third page, you configure the options table join. in this case, that is the column that joins the instruments table. Select the instrumentID column, set the type to number. The options form field should already be set to the instrument checkbox.
you will not need to make any changes on the fourth step.