First of all, your English is just fine, so no need for any apologies. We are also not Englishmen here. :)
Now a few things that you must know before going for a conversion:-
1) Warranty wil be void ( you know that already)
2) Make sure if only u r usage is more than 2000 kms. per month.
3) The power of the car will decrease.
4) You must get the change mentioned in your Registration Certificate
You can use any RTO licensed LPG Fitting Company for the conversion. I recommend Longas or Lovato. But make sure you get an original one. There are many duplicates in the market. So buy from a trusted and genuine reseller.