Use a machine with plenty of free disk space, preferably 800MB for a "whole hog" installation, plus space for data. Unwanted packages can always be kicked off afterwards.
Check the manual and online listings to make sure that the main elements of your system - hard drive, graphics card, network card - are supported by Linux.
RTFM (read the "flipping" manual), or rather "read it and do exactly what it says" in relation to noting down the components and settings of your PC before starting.
Backup important information on the PC before starting.
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Work methodically, stopping to take notes and read warnings carefully. Charging on in a fit of enthusiasm will probably mean having to do the whole thing again.