diff --git a/HowToWriteTests.md b/HowToWriteTests.md
index 4cab9d7..7d76665 100644
--- a/HowToWriteTests.md
+++ b/HowToWriteTests.md
@@ -1,3 +1,5 @@
+# create a test file
+
 Choose a name for your test file. We'll use `test_fname.py`:
 
 ```
@@ -8,3 +10,32 @@ touch src/allmydata/test/test_fname.py
 ./bin/tahoe debug trial allmydata.test.test_fname
 ```
 
+Okay, so it didn't work, because there is no file by that name. Create such a file, with these contents:
+
+```
+rom twisted.trial import unittest
+
+class T(unittest.TestCase):
+    def test_a(self):
+        pass
+```
+
+Now install Ned Batchelder's "[coverage"](http://nedbatchelder.com/code/coverage/) tool and run your with code coverage, like this:
+
+```
+./bin/tahoe @coverage run --branch --include='src/allmydata/*' @tahoe debug trial allmydata.test.test_fname
+```
+
+If you installed coverage from Debian or Ubuntu then you have to name it `python-coverage`, like this:
+
+```
+./bin/tahoe @python-coverage run --branch --include='src/allmydata/*' @tahoe debug trial allmydata.test.test_fname
+```
+
+This does the same as running the tests without coverage -- print a list of what happened when each test was run. It also writes out a file named `.coverage` into the current directory. Run the following command to read that file and produce nice HTML pages:
+
+```
+./bin/tahoe @coverage html
+```
+
+That will product a directory named `htmlcov`. View its contents with a web browser.
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