Link to a site that details the installation:

http://www.vwtrendsweb.com/tech/0404vwt_westy/

After replacing mine, I'd recommend taking the top off, but some volks leave
it on and do it.
I'd also recommend getting a rechargeable screw driver. I used a
Black/Decker and it saved a lot of time and made the job much more
convenient, especially in the tight corners. My van is an '87 so it uses
screws all the way 'round, not the wood strips mentioned in the above link.
The process for being sure the canvas is centered is the important part. I
just centered the top front of the canvas in the middle of the pop top, put
that screw in first then stretched it to both corners and set those screws.
I made sure to noticed how the aluminum strips came off so there was no
confusion putting them back on.

It took me about 8 hours. A local van friend, John C. helped me get the top
off/on, and that made a big difference. Being able to work on the top while
it sits upside down on a couple saw horses made this job much easier, I
think. You can pull the corners tight at the front much better while it's in
that position. Can't imagine doing that while the top is on the van, or
getting to the top screws at the rear. Very tight space to get a driver of
any kind into, let alone your hands to turn the screws.

I'm pretty sure you're going to get instructions with your canvas. The
instructions that came with mine worked great.

just my $0.02

Bob