Here's a list of things to check before you get started.
Disk space needed: from about 100Mb for a basic GHC build, up to probably 500Mb for a GHC build with everything included (libraries built several different ways, etc.).
Be sure that the “pre-supposed” utilities are installed. Section 6 elaborates.
If you have any problem when building or installing the Glasgow tools, please check the “known pitfalls” (Section 11). Also check the FAQ for the version you're building, which is part of the User's Guide and available on the GHC web site.
If you feel there is still some shortcoming in our procedure or instructions, please report it.
For GHC, please see the bug-reporting section of the GHC Users' Guide, to maximise the usefulness of your report.
If in doubt, please send a message to <firstname.lastname@example.org>.