Save all of your stock original parts in case this whole thing goes wrong, so you will at least be able to have a working rifle in stock form.
The plastic handguard should rotate into the grooves of the sheet metal on the ends if the gas tube. So you would set the handguard, basically upside down and then rotate it until it is line up properly. Sometimes some material has to be filed off of the handguard to get it to fit in the grooves. Do not file on the metal if it can be avoided for installing the handguard onto the gas tube. If the gas tube needs to be fitted to the rifle, then you can file a bit off of the gas tube assembly, but don't file the rifle. The basic rule is file the most replaceable part.
My personal rule with SKS rifles is to keep them as close to stock as possible.