Drop the forks out on the clamps and unscrew the top. Be careful as the spring will have a tiny bit of load on it and it will pop off, so as your undoing the last bit by hand put a rag over the top and hold the rag down on the leg with the other hand.
Once its off, pull the spring out and put the bottom in a cup and let the attached oil drain off.
Look down inside and you should be able to see the oil level.
Get a steel rule and poke it down inside to measure the level from the top (when fork is at full extension)
Tip it upside down and drain it off, you will need to 'stroke' the fork a few times just to be sure all the old oil is out.
Then tip it the right way up, and fill it up, once the oil is back up to the level you measured with the old oil, 'stroke' the leg a few times more to make sure oil is through all the valves and to remove any trapped air.
Check you level again and top up or remove as necessary.
To remove oil I'd suggest getting a 50mL syringe and carby overflow tube attached to the end to suck it out.
Let the fork sit for a few hours as there may still be a bit of air trapped.
Put the spring back in and screw the top back on.
Tip: If you can, measure how much you oil you put in. as if you have the volume right, next time it will make it much faster to change oil and you will know how much should be in there and if you have lost any.
From here you will need to go out and test ride, if its to soft add some oil. too hard take some out.
If it rebounds too fast add high weight oil, if its too slow, add a lighter oil.
You can mix oils too if you are finding that the oil just isn't quite right . Say 50% 5w and 50% 10w you get a 7.5w oil.
You will no doubt have to go through this process even with a new set of forks that have all of the adjusters on them. As with a china engine, the oil in the forks is S**T!
