VBA - using Checkbox's to fill in the table

I'm a little experienced with VBA but I stucked with the problem of using Checkbox's and I cannot to move on, as its a first time I prepare something using this element of UserForm.

SITUATION: First thing is that I have a table, which more less look like below:

CLIENT1 CLIENT2 CLIENT3 CLIENT4 CLIENT5 Question1?
Question2?
Question3?
Question4?

so I have several clients in first row and several questions in a first column.

PROBLEM: I would like to fill in the table with the Initials, which I inserted at the beginning of the macro thru MsgBox, but I would like to do this way:

When I open the macro I'm inserting initials, then I have a UserForm with Checkboxes to select the Clients, which I'm interested, so it should be Client1, Client2 etc. Then I select one client or several clients and press OK and then I have a second UserForm with "Question" checkbox's. I selecting the questions and press OK and then cell related to the Client and Question should be filled in with initials, if not with N/A.

maybe you guys could help me with that?

thanks in advance!