There should have been a page of Filing Instructions that printed out with the return. Basically you send the Form 1040 (or Form 1040-SR), and the forms and schedules that have an attachment sequence number in the top right corner. You do not send any worksheets or summary pages. Worksheets will usually say "for your records" (or similar) at the top and won't have an attachment sequence number. Also attach any informational forms as required in the paragraph below, such as those showing any withholding.
NOTE: It is a good idea to mail the return by a method that has tracking such as Priority Mail or Certified Return Receipt. If you also have a state return, the state return is to be mailed to the state tax agency. Do not make the mistake of mailing both to the IRS.
The below is a paragraph pasted from the actual IRS Instructions for Form 1040:
"Assemble Your Return
Assemble any schedules and forms behind Form 1040 or 1040-SR in order of the “Attachment Sequence No.” shown in the upper-right corner of the schedule or form. If you have supporting statements, arrange them in the same order as the schedules or forms they support and attach them last. File your return, schedules, and other attachments on standard size paper. Cutting the paper may cause problems in processing your return. Don’t attach correspondence or other items unless required to do so. Attach Forms W-2 and 2439 to Form 1040 or 1040-SR. If you received a Form W-2c (a corrected Form W-2), attach your original Forms W-2 and any Forms W-2c. Attach Forms W-2G and 1099-R to Form 1040 or 1040-SR if tax was withheld."
You probably realize this, but if you printed out a two-sided form (like Schedule C, D, etc.) on separate pages, be sure to send in Page 2 of the form as well even though the "attachment sequence number" is only on the Page 1 of the form.