The first entry in Mostyn Williams’s diary is very short and does not appear to tell us much about Williams.

In army 1 year. Preparing kit for departure

It is, however, possible to deduct from the post that he must have joined the army very soon after the outbreak of war. The United Kingdom declared war on Germany on August 4th 1914, and thousands of young men signed up. They needed to be equipped and trained before they were sent to the front, and that took time.
Williams was in the Welsh Regiment, 11th Battalion. The 11th was a service battalion. Although members of the service battalions were given some combat training and could fight in a primarily defensive role, their main role was to provide vital logistical support. That is reflected in Williams’s diary; when he mentions his work, it is often a question of digging and making roads, in particular in the beginning of his posting abroad.
BBC Wales features an article about the 11th Battalion of the Welsh Regiment, also known as the Cardiff Pals Commercial Battalion.
http://www.bbc.co.uk/blogs/wales/entries/34332e6e-74a3-35a3-8c88-8ffec0edb42d