model = nn.Sequential(
    nn.Linear(100, 50),
    nn.Linear(50, 20),
    nn.Linear(20, 10)
)

This is the easiest way. The nn.Sequential object will collect all the modules that you pass to it and when it call its .forward() method it will feed each layer into the next. If you need to define a more complicated forward pass you can override the forward of an inherited nn.Module:
class CoolModel(nn.Module):
  def __init__(self, *args, **kwargs):
    super(CoolModel, self).__init__()
    self.layer1 = nn.Linear(100, 50)
    self.layer2 = nn.Linear(50, 20)

  def forward(self, x):
    out = self.layer1(x)
    out = self.layer2(out)
    return out

Hope that helps!