The stock spring on the CRF250L is extremely soft. If you weigh over 120lbs (which you probally do..) you will want to load up the pre load. turning up the pre-load will keep more tension on the spring and making the suspension a little stiffer and better for us non child riders
Start by jacking up the CRF250 or putting it on a stand to take the pressure off of the spring.
If you look on either side of the bike you should be able to see the spring under the seat. The two rings on top are how you adjust the preload. The bottom ring is the adjuster and the top ring acts a a locking nut.
You should be able to adjust the rings using a large flat head screw driver and a hammer. Don’t worry about being to gentle as these rings will take some force to break free and spin. (don’t worry they are tough.. give them a few hard smacks!)
Standing on the right hand side of the bike you will want to spin the top ring forward to loosen the nut. Once loose you are free to adjust the bottom pre-load ring. Smack the bottom ring towards the back of the bike (when standing on the right side) until its reaches the bottom of the threads. Looking from down below you should be able to see how many threads are left.
Once you have the bottom ring all the way down (or to where you want it), spin the top ring towards the back to tighten it back up and lock the bottom ring in place. Put your tool away, go for a ride and enjoy your slightly stiffer suspension.