I was about to write up a long screed on how to do this effectively but then thought about Fibrecrafts.
I have isssue with their quantities of mordants, a very little goes a very long way and I'm not in business to selIl the stuff, so I suppose it's inevitable we may disagree.
However, their instructions and their products are very good.
I will not use tin or chrome as a mordant. One forms stable organo metallic compounds that are persistantly toxic in the environment and the other causes feminisation of invertebrates at the bottom end of the foodchain.
I get a really good range of colours from natural dyes without toxifying my environment, I'm not using those two.
Their link
http://www.fibrecrafts.com/resource/fact_file/dyes/mordants.asp
Linen is a cellulose fibre so instructions for dyeing those will be relevant.
If all this throws your plans,
www.kemtex.co.uk carry an excellent range of dyes for home use too.
cheers,
Toddy