diff --git a/_episodes/05-performance.md b/_episodes/05-performance.md
index 7837144..ac06476 100644
--- a/_episodes/05-performance.md
+++ b/_episodes/05-performance.md
@@ -225,6 +225,12 @@ times to observe how much variance there is.
 
 #### Compiling and Running the Example
 
+0. Connect to a cluster (if you haven't already), with X11 forwarding enabled so that you'll be able to display some graphs.
+   - If you're on Windows using MobaXterm, check that the "X" icon in the upper
+     right-hand corner is colored.
+   - If you on a Mac, start XQuartz, then start a terminal and log in with
+     `ssh -X username@cluster...`  The uppercase `-X` turns on X11 forwarding.
+
 1. Download and unpack the code:
     ~~~
     wget https://acenet-arc.github.io/ACENET_Summer_School_General/code/julia_set.tar
diff --git a/setup.md b/setup.md
index 6d6b6c5..32fa13f 100644
--- a/setup.md
+++ b/setup.md
@@ -24,10 +24,15 @@ Although macOS comes with SSH pre-installed, you should install
 [XQuartz](https://www.xquartz.org/) to enable X11 graphical support.
 Note that you will have to restart your computer to complete the installation.
 
+When you connect to a cluster, first start XQuartz.
+Then use `ssh -X ...` to enable X11 forwarding.
+
 **Linux**
 
 Linux users do not need to install anything, you should be set!
 
+When you connect to a cluster, use `ssh -X ...` to enable X11 forwarding.
+
 ### Optional Do-It-Yourself requirements
 
 If you want to carry out the exercises on your own equipment you will need: