visa4uk for parents .. please help

[Terpe]

.....Do I have to send first all the documents to my parents in Philippines before filling up the application form online.....

You parents will be the applicants and they will need to attend and submit the paperwork at the VFS Application Centre (Manila or Cebu)
Once the application has been submitted online an appointment will need to be made with VFS

It's really a question of timing and personal choice.
Probably the safest route is to send all the needed documents once the applications have been submitted.
The main concern is making sure that payslips and bank statements are not older than 28 days on the date of application (date of visa fee payment).
So the appointment at the VFS application centre (booked online) needs to be timed for that.

.... My parents might find using the computer difficult in filling up the form. Can I possibly fill up and complete their online application form and just send to them the printed application form for them to sign in...

Yes, that's allowed for. The online form will ask if you are filling out the form on behalf on someone else. No problem at all

....Lastly, regarding the online payment. My parents don't have any visa cards or mastercard. Can I just pay for their visa application using my own card....

Yes, no problem at all. That's virtually standard practice these days.