Mention location and we will be able to guide you better.
For Lahore now a days (till 30th of this month) you need to get a TO form from excise office; window 7. Fill it up and submit it in a file with sales deed and original book. TO form is good for 20 days or so, the old one may have expired and you will need a new one.
I don't recommend you to forge the signature and the thumb print if the seller (ahm ahm, that is something everyone does, don't ask me if I did that too). Go to window 4 for transfer, there the guy will check your original file and accept the transfer file. Transfer file must contain the following:
Sales deed,
TO Form
ID cards (both buyer and seller)
Original book
You will need to use a string that comes with the file to clip all these together. Pay the fee and get the receipt. In 4-5 days you will be able to collect the file from second floor.
I didn't pay an agent and got the transfer done for my car for 1700. (300 for TO form, 1200 excise fee, 200 transfer fee). Ek pesa nae dia agent ko or kam bhee aram say ho gaya, bus window kay ek do chakar lagay, reception pay jo ladies bethi hain they were really helpful, just give them the respect they deserve.
From 1st May you will need to have the seller and buyer both at the excise office for a biometric transfer. So open letter can be a problem since it will be your responsibility to track down the seller and bring them in for boimetric transfer.