To invite your parent you should have a good reason/celebration so that they can explain to the visa counselor in Nepal. All you need to do is send then a letter stating you really wanna invite them for .....reason. After that, everything is done from Nepal. They need to arrange all the documents like - Financial, property valuation, relationship, bank statement and others.

Finally, with all the paper work ready, need to fix a date for interview and if everything is genuine than Counselor will issue the Visa.

P.S. - Try applying them for a very good reason,

Good Luck !!