How do you attach the nylon fuel line to the steel fuel line?

That cannot be done in most cases, you are suppose to replace the line but lets give it a shot.

When you say. "Nylon Tubing" I assume you mean a braided hose made of nylon. Just want to make sure as not using a hose that is not rated for fuel is dangerous.
What I have found to get a smaller braided hose onto a larger steel hose is to get a large Phillips head screw driver, a little smaller than inside of hose, and work it around prying it side to side until you get it to stretch.
Have you compared the inside diameter of the new hose to the old one? Make sure that they are about the same as that will cause a pressure back up if the fuel line is too small.

Also, try spraying silicone or out some motor oil on the steel tube to get it to go on. A pair of pliers is helpful if to get a better grip on the hose to push it on sometimes.
I hope this helps.

The smaller line will end up stretching over the end of the steel line as I am assuming it is flared. So, it will be stretched anyway. Nylon line is not very forgiving especialy if it is the rigid type.
The biggest thing is that the inner diamters of the 2 lines should be the same or flow characteristics might change and pressure will increase in the smaller diameter line. Burst strenth should not be an issue as it is very high in nylon lines.
If it not a semi-rigid or somewhat flexible, I do not reccomend using it if it is going to have any sharp curves in a short distance. It can kink and casue a blockage. Nylon fuel line is generally rated at thousands of PSI, for fuel line, make sure you get the proper type as some nylon does not have the burst strength for fuel injection systems. The issue that concerns me is the proper attachement of the 2 lines to each other and getting the end compressed on correctly. Nylon tubes usually use a compression fitting with a cap nut and ferrule.
Flare nut and cap fittings are cheap and a tubing flare end kit can be bout for less than $20. Fuel fittings are a double flare 45 degree fitting. Some kits come with 37 and 45 degrees in case you think you might ever make a stainless steel braided line with AN fittings. The 45 degree kit will cover the repair of most flare fittings on your car. If you have a tight bend to make, use a pipe to bend the tube and save the $10 on a tubing bender. You might want to buy a tubing cutter, which I have seen for less than $10.
If the line was steel before, you should replace it with a steel line as I have never heard of special clamps to hold extreme pressures and it is just not the way to repair something that is safety related. The change in pressure could be enough to blow the lines apart. You can find metal to plastic compression fittings, but you will have to cut and flare the steel line in some cases. The fittings are available through SummitRacing. Com. I looked for them but could not find them, summits web site is huge, I am pretty sure they exist. The Tech line number for Summit is, 1-330-630-0240. They will be able to find them if they are still produced.
I still don't reccomend them as I am a stickler for not using patches to fix things. I do not have the experience with the conversion nuts to give you an answer or reccomend them and be confident in my advice.
Please let me know if you have questions, because it is a lot of information. I just wanted you to have all the facts in front of you so you can decide what to do.