I am very interested in building a fixed gear bicycle. The base bike I have is and old fugi sports 10spd with a freewheel. I want to olny build this bike out of the parts I have (I have ALOT of old road bikes around). What is a good gear ratio to make it?, (It is fairly hilly around here). And am I supposed to screw a cog right onto the hub threads. Other than that I would greatly apreciate tips about building and riding fixed gear bicycles.
I have already visited this page:
https://www.sheldonbrown.com/fixed-conversion.html
Just wanted to see if I could clear some things up

head first brother buy a wheel and some cogs and beat the buisness out of it . youl learn where your comfortable or die trying. dont bother w a brake and rock some platform pedales too. now just do a post about what bag is right for me?

Ride the bike around staying in just one gear, should give you a sense of what gear ratio you need. If its hilly you're going to have to find a compromise between being able to get over the hill and then down the other side without spinning out madly (running brakes will help you avoid spining out).

Sometimes there is no easy answer to this gear ratio question and you'll just have to try a number of gearings before you find something comfortable for you.

I'd start with something light such as 42x16 (approx 69 gear inches), which worked great for my girlfriend in SF.

Oh and read these things too:
https://www.63xc.com/gregg/101_1.htm (Fixed Gear 101 Guide)
https://www.superdrome.com/docs/pumabook101.pdf (Puma sponsored fixie guide, has some good advice nevertheless)

Remove the freewheel, add a track cog, make sure you secure the cog with a lockring, you can also use something like lock-tite and JB Weld and your fine. I don't have a lockring on my conversion and I have been riding for months.

JB Weld isn't such a good idea if you want to change cogs at some point. Red loctite needs heat to remove. If you're really worried about unscrewing your cog, use that and take a propane torch to the hub for a minute when you want to switch.