Create Userform That Adds Integer Data To Existing Data In Specific Cells

I am trying to create a user form that will allow the user to type int values in boxes. Then when the user clicks the submit button the code needs to add the values from each user input box to the existing values in specific cells accross multiple worksheets. Then the form needs to be cleared after the cell values are updated. I can create the form it is the code on the submit button i am lost on. Also it is important that some boxes may be left blank.

If you click the Grey "Qty Form" button on sheet1 the form will open. User data numbers can be entered in the blank boxes. When the submit button is pressed the form needs to add the user entered numbers to the numbers in the corresponding cells in sheet 1 and sheet 2. How to code the submit button to do this properly. Also after the data on the spreadsheet is updated the form needs to be cleared and start the cursor back in the Item 1 box on the form.