The standard CRF300L (non Rally model) comes from the factory with handlebars that have threaded inserts in the ends, so this means installing handguards on them will be a little more difficult than with a standard hollow end handlebar. Most aftermarket handguards come with insert kits that slip right in the ends of a standard hollow handlebar, but that threaded insert in the CRF300L's bars is in the way. And unfortunately the long bolts that come with handguards are too large to fit those threaded inserts, so you have a couple options (see below).
The stock CRF300L handlebars have threaded inserts that will accept an M6 x 1.00 bolt. Unfortunately many aftermarket handguards, like the Zeta Armor handguards we wanted to install, come with insert kits that have larger/stronger M8 x 1.25 bolts. So, you can't simply thread the handguard bolts into that insert. On our CRF300L project bike, we wanted to try and use those larger bolts that come with the Zeta handguards, so we decided to drill and tap the handlebar inserts so that they would accept the M8 x 1.25 bolts that come with handguards. The chart above gives you an idea of what size drill bit to use to drill them out, and the what size tap to use so that the handguard bolts will fit.
Once we drilled and tapped those inserts, we were able to install the Zeta Armor handguards using their included M8 x 1.25 bolts to attach the handguards to the end of the handlebar, and then used the included 7/8" handlebar clamps to attach the in-board part of the handguard to the handlebar. We also installed a set of the Zeta Adventure Armor handguards, which have slightly different style handlebar clamps than the regular Armor handguards. Both fit the bikes stock bars fine, but I think the standard Armor Handguards fit a little bit better overall.