Once you've got that open, usually anyone with common knowledge in producing could figure it all out but you being a novice, I'll throw you a few custom variables for you to mess around with and find what you like: If you grab Audacity or already have it, you can start by finding 'Generate' in the top column and select 'Chirp' or 'Chirp Generator' in the listing. Literally all an 808 is, is an initial deep sine wave that usually has some sort of compression / distortion or EQ'ing done to increase volume and low end and more then usually faded out. It isn't hard at all and could be done in third party freeware such as Audacity.

Considering how 808's aren't something you could 'stretch' or duplicate without some muddy low end and an overall crappy 808 pattern, you're going to have to resort to making your own OR digging for new samples that fit your needs. Personally, I don't really touch 808's too much but with some generic knowledge in how they work and are created I can only suggest that you make you own to your specific needs. We'll, whatever the skill level, you're always welcomed here at FutureProducers.

808 in fl studio