The Clymer manual for the earlier XT600's is also relevent to your bike in many areas and covers this.
Briefly, the basic drill is when the engine has not been run for quite a few hours (IE: left overnight) :-
First time doing this, remove petrol tank for better visability / easier access
Remove spark plug making sure no dirt falls down it's hole & into engine (again, not essentiol but makes turning engine over by hand easier, I wouldn't do this if spark plug tunnel has dirt in it and air line not available to blow it out)
Remove the large round cover (that is near your left ancel when riding) & the small one just above it. Use a large socket on the nut on the end of the crank that is now exposedand rotate the engine ANTI-CLOCKWISE ( I think, best double check) and look through the small hole for timing marks on the rotor as it passes. These have varied over the years, but you should see 2 or 3 lines pass behind the hole, the last one is TDC and may have a T stamped to the left of it ( on my 88 tenere it's badly stamped and looks like another line. The other tines are for ignition timing)
Position the engine at TDC
Again making sure no dirt can fall in, remove the inspection cover that's above the carb inlet stubs and the 2 inspection covers above the exhaust headers.
You should now see 4 rocker arms, Check to se if they can all rock up and down slightly. If not, rotate the engine one ful turn and recheck.
Use feeler gauges to measure the gap between the bottom of the adjuster screw that is in the end of the rocker arm and the top of the valve that it pushes on. You are looking for a bit of drag as you slide the blade(s) through the gap, but no a lot of fource.
Remember that the required gap is different for inlet and exhaust,
Hopefuly Fred is right and you will verify no adjustment necessary.
If not, use a ring spanner to loosed the loc nu that secures the adjuster screw (they round off if you're not careful) and screw the adjuster up or down as required, re-tighten loc nut, find thst retightening it has changed the setting and try again !
There are tools that help here, but it can be done with a ring spanner, allen key & patience.
Small, angled feeler gauges also make it much easier but are not esential
Replace engine covers, cheching o rings are correcly located. I put medium strength loctight on the screws and lower covers, partly to prevent dissimilar metal corrosion between engine alloy and steel screws, but it's not essential.
Hope that helps