diff --git a/icu4c/source/data/Makefile.in b/icu4c/source/data/Makefile.in
index 3e917a8812d..778b6c7affa 100644
--- a/icu4c/source/data/Makefile.in
+++ b/icu4c/source/data/Makefile.in
@@ -317,7 +317,9 @@ $(OUTDIR)/icu4j/icutzdata.jar: generate-data
 # - extract all data items
 # - package them into the .jar file
 # TODO(ICU-20466): Move this to Python
-$(OUTDIR)/icu4j/icudata.jar: generate-data
+# NOTE: If you get the error "No rule to make target 'out/build/icudt64l/uprops.icu'",
+# you need to reconfigure with unicore data.  See icu4j-readme.txt
+$(OUTDIR)/icu4j/icudata.jar: generate-data | $(OUT_DIR)/uprops.icu
 	$(JAR) cf $(OUTDIR)/icu4j/icudata.jar -C $(OUTDIR)/icu4j $(ICU4J_DATA_DIRNAME)/
 
 ICU4J_DATA=$(OUTDIR)/icu4j/icudata.jar
diff --git a/icu4c/source/data/cldr-icu-readme.txt b/icu4c/source/data/cldr-icu-readme.txt
index 23cf0a91476..d162fd70ba0 100644
--- a/icu4c/source/data/cldr-icu-readme.txt
+++ b/icu4c/source/data/cldr-icu-readme.txt
@@ -241,11 +241,21 @@ cd $ICU4J_ROOT
 ant all 2>&1 | tee /tmp/icu4j-oldData-antAll.txt
 ant check 2>&1 | tee /tmp/icu4j-oldData-antCheck.txt
 
-# 12. Now build the new data and test data for ICU4J
+# 12. Transfer the data to ICU4J.
+# 12a. You must first configure ICU in order to add the unicore data.
+
+cd $ICU4C_DIR/source
+ICU_DATA_BUILDTOOL_OPTS=--include_uni_core_data ./runConfigureICU Linux
+
+# 12b. Now buil the jar files.
 
 cd $ICU4C_DIR/source/data
+make clean
+make -j6
 make icu4j-data-install
 cd $ICU4C_DIR/source/test/testdata
+make clean
+make -j6
 make icu4j-data-install
 
 # 13. Now rebuild ICU4J with the new data and run tests: