Grinding lenses is pretty much like grinding mirrors. Newport Glass has a nice page on the web about the problems of putting two curves on a piece of glass. The biggest problem with lenses is making sure that they don't have any wedge in them. The second biggest problem is getting the thickness of the lens right. I might also note that you should raytrace out the design before you actually start grinding to make sure that the design is what you are interested in. Willmann-Bell has a book with software for designing telescopes for a price of $50 ($25 for book and $25 for software to go with the book) and if you are doing a basic Fraunhofer or Clark design then I would suggest that you get the book and software and peruse the design and what else you can do. You should be able to do a lot better (at least half the errors) than the stock numbers that the basic designs do. Have fun with the project. Even if the scope isn't perfect, it will be something that you can really say you did yourself.