To start off, you can have three PCs in your lab with PC1 running the application server, while PC3 runs the app client. In-between is PC2 where NetSim emulator runs simulating congestion/delay/loss etc. All three are connected to a common L2 switch. The PC running NetSim must be Win 7 (SP1 or higher), Win 8 or Win 10. The files downloaded by the client from the server will 'flow through' the network emulator where they will experience various networks effects. 

Note that these can either be PCs or VMs, and if you want multiple clients then you just need to increase the number of PCs/VMs. Each real PC/VM will be mapped to a virtual node inside the emulator. If you want many numbers of clients then it is usually done by having a few real clients and many 'virtual clients' inside NetSim which will generate background traffic. 

You can then have real downloads from the server to the 'real clients' and analyze the performance with varying levels of delays/loss/congestion etc.