Some sealant like Stans, CaffeLatte or JRA Wheel Milk.
Get a tyre that says tubeless-ready on the side for a best results although you can get good results with any tyres that have sturdy sidewalls to minimise burping, and a tight bead to aid inflation. If your tyre fits very easily and is flimsy, expect trouble down the trail. YMMV.
First put some soap suds on the rims and fit the tyre. The beads on tubeless ready tyres might be difficult to stretch onto the rim, which is a good thing, so use tyre levers. Inflating with a track pump should be easy if you used tubeless-ready tyres. If it isn’t, more layers of tape in the rim helps take up the slack and aids inflation. Once the tyre is nicely on at 60psi and hissing around the bead, just let it deflate.
Now unscrew the valve from the valve stem with some pliers and squirt plenty of sealant in with a syringe (buy from Boots), replace the valve and re-inflate.
Hold the wheel upright, slosh back and forth vigorously, rotate it a bit and continue sloshing, going full circle, or until you think it’s stopped leaking. Leave it lying on one side. It might go down after a while, in which case just repeat the sloshing and lying down until it stays up.
Set the tyre pressures you want for riding. As a rule-of-thumb, drop 5 psi from what you usually use. With sturdy tyres you can go surprisingly low which all helps improve grip, comfort, compliance and reduce rolling resistance on the trails. Too low and they will squirm in corners and peel off the rims with a horrid burp, possibly sending you into the bushes. It’s trial and error.
Perhaps 3 weeks after initial tubelessing, you might need a sealant top-up. The main thing is to get used to sloshing the wheels before a ride to make sure they never dry up completely.