Ok..Iv'e had a go at making a quick tutorial for you for a simple arch.
Your dimentions may be different but the concept is the same.
Ok drag out a 64x64x192 block.

Now go to
curves---->primitives---->square cylinder

Ok now change to front view

Ok now select your block and then press
Ctrl B and you will see a series of dots come up.

Press
enter once and the blue dot should appear in the middle of the pink ones either side (if it isnt press
Tab until it is.

Now press
Enter then Tab until you get the 3 blue dots at the top as shown below.

Right now hold down your left mouse button to the left of your block and drag it up to bend the block so it looks like this below.

Press enter and deselect...by now hopefuly you have one half of an archway.
Ok now select the arch again and press Space bar to duplicate it, press the
ZN (z axis rotate) to align your 2nd arch piece with the 1st using all views to assure correct alignment.

Ok now you have your arch but it may seem a little small in height so just simply add on a couple of blocks either side to extend it using the same measurements.

And there you should have a perfectly acceptable arch, you can mess vith vertices etc to make it more elaborate but you should get used to doing this 1st.
Now texturing...hmm pain in the ass is texturing an arch ,hit the surface inspector and either try
Natural (sometimes you have to press natural twice, or bring down the scale of the horizontal and the virticle by reducing each by one click. (dont be alarmed that there is nothing in the scale box it still works)

Dont be put off if in radiant it looks like the texture on your arch is darker/lighter...ingame it will look fine.
This is my 1st attemt at a tutorial and I havent taken long to do it so I hope its all correct and it helps.
Seere.