Using DOSBox is pretty similar to using a real DOS PC. The one thing to keep in mind is that you have to actually "mount" a folder from your PC, which will then make it look like a drive to any game in DOSBox.
Let's say for example that your copy of Lemmings 2 is in "C:\Lemm2\" (In practice, it's more likely to be somewhere like "C:\Users\[your username]\Games\Lemmings2\" or something like that, but we'll use a short folder for the example).
You'd enter this command:
mount C C:\Lemm2\Once you've done this, within DOSBox, the folder that's actually C:\Lemm2\ on your PC, will appear to be just the C:. So from there, you can simply enter:
cd C:\L2-FIXAnd that should get you into the game.
If you find the game is laggy, you may need to increase (or sometimes, decrease) DOSBox's "cycles". Use Ctrl+F11 to decrease, Ctrl+F12 to increase.
Once you get more familiar with it, you can set the default cycles in the config file, and perhaps add your mount command to the AUTOEXEC section at the end. Don't worry about that at first, though.
For the record, while you'll need DOSBox (or emulation of a different system) for L2, L3 and L3D; if you're wanting to play the original Lemmings (or games based on it - eg. Oh No! More Lemmings!, Holiday Lemmings) at any stage, I highly recommend using Lemmix, Lemmini or NeoLemmix instead. They're much more convenient, and work directly on Windows.
In particular, Lemmix replicates the DOS versions almost exactly, even including glitches (despite the name similarity, NeoLemmix does
not; the name comes from the fact that it's forked from Lemmix's source code).