I am a intermediate-level player. How can I proceed after a declined Queen's Gambit, as White?
There are multiple options here. I'm not sure what you mean by "an intermediate player", so the amount of specific lines you'd like to study can differ. I'll focus instead on types of pawn structures and middlegames you can reach. Let's take the reference position 1.d4 d5 2.c4 e6, even though there are different move orders available:
Black will sometimes play an eventual ...c5. This can lead to two types of pawn structures: the Semi-Tarrasch type, where you get pawns on e4 and d4 (see for instance 3.Nc3 Nf6 4.Nf3 c5 5.cxd5 Nxd5 6.e4). This often leads to a comfortable position for White due to his superiority in the center and not enough counterplay chances for Black to compensate for it. Here you have an example from top level players.. This line has some similarities with the Grünfeld Defence (see 1.d4 Nf6 2.c4 g6 3.Nc3 d5 4.cxd5 Nxd5 5.e4 Nxc3 6.bxc3), but I'd say Black has more chances in the Grünfeld. In my opinion you should try to reach this pawn structure as often as possible.
Another typical pawn structure that is often reached is one with an isolated "d" pawn. You'll probably play both with and against that isolated pawn in several occasions, you that's also something you may want to get familiar with. chess.com has an article about it that you may want to check out Note that this structure can also be reached when the Queen's Gambit is accepted.
Then there's the "minority attack" type of game. This happens when you capture on d5 and your opponent retakes with his "e" pawn but ...c6 has been played instead of ...c5. Here you can play on the queenside trying to get your semi-open "c" file fully open. I'm not really well-versed here so I'll let someone else choose an illustrative example by editting this answer.
Finally, there are some lines where Black will enter some variation of the Slav Defence (with ...c6). I don't know how well you know the lines after 1.d4 d5 2.c4 c6 and your preferred move choice can chance according to that. For instance, you can try 3.Nf3 Nf6 4.e3 so now you can answer to 4...c6 with 5.Nbd2!? if that's the particular line you enjoy against the Slav (here your plan would be to make a quick e4 push). But if you're not interested in playing that then you may prefer 3.Nc3. There are several lines you can choose from and plenty of transpositions between them.
Matthew Sadler's book Queen's Gambit Declined has received very high and widespread praise. Again it depends what you mean by "intermediate", but the book contains a ton of instructive games, and its clear and deep discussions of ideas should be helpful to players of a wide range of strengths.