Writing a COFF object file... how to reference external symbols

I'm going to write my first compiler (ok... most of the work was done by using ANTLR, but it still required effort)... I have already done the following things:

• Implemented a working interpreter for very, very simple calculations (+, -, *, /, ^ - with proper precedence, of course, which was not that hard to implement)
• Created a basic assembler code emitter (which was actually almost easier than writing the interpreter, because I used a rather simple "stack" based approach to handle the calculations - like: "push var1, push var2, add" - which could be then easily transformed into simple mov / add instructions
• Was able to assemble to .exe (using ml64) and execute the resulting executable

Now my question is - if I want to create the COFF object files myself, how do I reference external symbols in libraries / other object files?

I mean, for example (if I want to call printf) it works like this, when I do it in ml64:

lea rcx, "address of string"
call printf

But what exactly do I write into the COFF file instead of "printf"?

Everything else is not THAT hard to implement, but I have absolutely no idea how to go about referencing libraries / other object files... hm...

Edit: fixed formatting (sorry) + clarifications

Instead of the printf, you write four times 0x00, i. e. the call itself is assembled as 0xe8 0x00 0x00 0x00 0x00. Further, you need to write a relocation record with type DISP32, with the offset pointing to the first 0x00 and a value pointing to a symbol entry with the name printf. Wikipedia has some links to documents describing the COFF format in detail.