Hello Iestyn,
The first answer to this question I think is having the burning desire within you to want to learn it. There are of course many other answers to the question I'm sure. My personal reason became clear to me about 6 years ago following quite a bad illness when I realised that there were a few of my long-standing objectives which had not been fulfilled - and learning to speak Welsh was one of them, so when my health returned I set about to make this right.
I enrolled in a Welsh class run by Prifysgol Morgannwg 1 morning a week - after 2 weeks I decided that my progress was going to be too slow, I decide to enrol in their intense course 3 mornings a week, this was what I wanted - After 4+ years I was getting along quite well in the class, but one day it dawned on me that I was lacking in confidence when it came to talking with people outside of the classroom - Then! I found the SSIW website and things began to happen, and I really mean happen - I began to talk with much more confidence, and to every one who spoke Welsh, not worrying at all if I made mistakes (which were/are many). My main test came in May this year when my wife and I decided to go up to North Wales for a weeks break. It was then I decided that each day I would have at least 1 conversation in Welsh - This is how it went, I would listen to the accent of the people around me and say 'wyt ti'n gallu siarad Cymraeg' and when I had the answer 'ydw' the talk would begin - By the end of the week I was totalling between 3 - 4 conversations a day, I was very excited, and gob smacked by my success.
What I now realise is - Going to a class, and following the SSIW course is the way foward. Well! for me it is.
To conclude: The SSIW will give you the confidence to use the language in an every day way, But you must put yourself in as many positions amongst fluent Welsh speakers as is possible where you will be forced to converse in the language - This is what I do now, and it is beginning to work - I realise that I still have along way to go to reach the level of fluency that I want - But each small step forward is so exciting .
It is also important to attend any group of learners who meet each week to use the language - In my case each Mon 7.00pm - 9.00pm in 'Clwb y Bont' in Pontypridd
Arlunydd