Technically, there are two different things: Seeding, and pre-growing. Seeding is just that... rubbing seeds (spores) of algae into a new screen so it can start growing faster. Of note is the fact that you can't really "seed" real red/brown turf, because it takes months to develop, and if you really did rub spores of real red/brown turf into a new screen, they would just die. Seeding spores is done with just any old algae you can get from your tank, since a new screen can only start by growing brown or green algae anyway. It will save you two or three days of growing time.
Start by washing your screen with soap and water to get any oils off, then rinse it several times to get the soap off. Then take rough some sandpaper and sand both sides well. Then take some algae (any kind) from your tank and rub it HARD into both sides of the screen. Then run some tap water over it, which will flush most of the visible algae off the screen. You don't want any visible algae remaining; you want the invisible algae spores to remain. And they will indeed remain, stuck into the small spaces created by the sanding. The reason you want to rinse off the visible algae in the sink is because if you don't, it will just wasn off in your tank and die, or it will just die on the screen; in either case it will cause a bit of yellowing (removable with carbon). So just avoid this by washing all the visible algae off the screen after you seed it.
First visible green growth specs will be by day 3. And it will start pulling nutrients in a week. The real red/green stiff astroturf like turf will take months, however. But all algae absorbs N and P, so there is no need to wait.