If you are wanting a postal vote for the election on May 6th you have until Tuesday 20th to apply. Although as I have said before don't leave it to the last minute apply early to give the registration time to query any issues there may be with your form.
20th April is also the last day to ensure that you are actually on the register to vote.
If you live in West Lothian council area you can visit their website here for more information. If you live in Falkirk council area you will want to visit here
Forms for postal/proxy votes can be downloaded:
But remember time is running out to get your vote secured in the way you want to.