It’s very simple; just think of the present first:
The things you are making.
Die Sachen, die du machst.
And then transform it into the past tense:
The things you have been making.
Die Sachen, die du gemacht hast. (or: machtest; uncommon)
The key here is that in English you need to take care of two things: the choice of present progressive (because it is a process you want to express) and the correct choice of past tense (simple past or present perfect?) which gives a slightly different nuance. ‘The things you have been making’ implies that they are done while ‘the things you were making’ doesn’t.
German makes neither of these distinctions. Instead, they are inferred from surrounding additional context. Thus, the exact translation is the one mentioned above, even though it is also the exact translation for ‘the things you were making.’
However, some colloquial flavours of German have a colloquial progressive form. (The Rhine area jumps to mind.) Therein, you could at least express the progressive aspect as:
Die Sachen, die du am machen warst.
If you want to imply the finished bit, you can only truly do that adding additional words:
Die Sachen, die du fertig gemacht hast.