Second, you will need lots of photo reference of Batman. It’s up to you what kind of likeness that you want. Batsuit in Dawn of Justice has the fake muscles look. I tried this look but the articulation was hindered. So I tried to bulk him just to give him the big and bulky look.
Third, you will need to do some sculpting. I'm not an expert on sculpting. Really am not. But if i can do it, so can you.
Ok, now where to sculpt?
First, the arm. Specifically deltoid (shoulder) and tricep muscle. The bicep is already good so no need to sculpt there. The upper hand (above elbow) is a little bit thin so add thin layer of clay on the inside and outside of the arm to build some mass. Just thin layer is enough. After that, add tricep muscle because the batsuit has visible tricep muscle.
For the deltoid muscle, just sculpt right in the middle section of the skeleton. Leave the upper and the lower deltoid skeleton to keep free articulation.
Second, the lat muscle (side body or browse in google if you don’t know what that is). This will help you get the V shape body. Just add a small amount of clay on the side to give the V shape look.
Do the mod until you get the desired look and when you're done, let the clay harden and you're all set.
What's next after the mod? Well put on the suit. Since the suit is made of stretchy material, you shouldn't have any trouble putting it on... Except for the body/torso part because you've added lat muscle.
Well, that’s all I did. Very simple. Now I have explained it. It’s up to you. For me, I think stock body is a bit too thin. Batfleck is bulky. If you compared it, he even looks bulkier than Superman. But that’s the purpose. They want a Batman that can beat up everybody, including Superman. So Ben’s imposing physic was very important.
That being said, others might prefer the stock body because it’s already good enough. That's cool. Again, it’s personal preference. But I want the Batman to look menacing.
Hope that's help.