Firstly, welcome to the forum
This was being discussed the other week, have a look
here, some ideas about build tables.
To ensure I had a true and level surface, I placed a spirit level (about 3ft long) across the table at all positions, both along and across, not forgetting the diagonals.
The lines should represent where your tubes will go (as a hint, most steel rules are 25mm wide
)
I used little bits of timber as the guides for the tubes, some people have used those little blocks for screwing kitchen cabinets together. An alternative would be to use some steel angle (although drilling loads of them out would be boring
Don't worry about asking questions, it's what the forum is for, but it may have been asked before, so remember there is a search function