In this case, it is largely up to you. There is no "right" answer.
Everything that follows is my favoured approach (which I am somewhat reluctant to post due to it being almost purely opinion). I should note that others may rightfully disagree.
That said, it seems to me best practice to describe each method separately. That is, your description might be:
Figure 1 is a sequence diagram showing a method performed between a client and a server. At event 101, …
Figure 2 shows a method performed at a client. At step 201, …
Figure 3 shows a method performed at a server. At step 301, …
For completeness, you would also tend to describe the hardware performing the methods.
Figure 4 shows a system 40 comprising client 41 and server 42 … In some embodiments, system 40 may operate according to the method of Figure 1. …
Figure 5 shows a client 41 in accordance with an aspect of the invention. … In some embodiments, client 41 may perform the method of Figure 2.…
Figure 6 shows a server 42 in accordance with an aspect of the invention. … In some embodiments, server 42 may perform the method of Figure 3.…
The key reasons for separating these out are clarity and basis. By making these painfully explicit, you are making the invention as clear as possible. Moreover, for jurisdictions which are touchy on basis (such as the EPO), you are providing a strong basis in the description for a claim directly solely to the method performed at the client etc.
