You will need valve oil, an old cloth and an uncluttered area to work. Follow the procedure one valve at a time until you are familiar with the process. There are several types of valve locating methods, some use a fixed guide at the top of the valve, some have a 'floating' guide with one locating pip and some have two 'floating' pips (some have two pips that are different sizes).

With some types of valves it is possible to fully insert the valve and screw down the valve cap but find the instrument is blocked when trying to play. The following method will work for all types of valves if done with care:

ONLY DO ONE VALVE AT A TIME

1) Unscrew the valve cap that is below the valve button.

2) Partly withdraw the valve and note the position of any numbers on the stem / spring cover. Later the valve should be put back with the number in this position. In the case where there is no visible number don't worry as the following method will find the correct position.

3 )Withdraw the valve fully and oil over an old cloth (note that valve oil stains clothes).

4) Partly insert the valve back into the instrument gently twisting so that the number is in the position noted when taking out. If there is no number on the valve just partially insert it into the instrument.

5) Gently lower the valve into the casing until it is as low as it will go. Rotate the valve back and forth just a little until it is heard to click. (If the valve does not click go to 5a). At this point it should not be possible to rotate the valve any more - do not twist hard - if this is the case skip.

5a) For valves that do not click in gently twist further until it clicks in - it may be necessary to nearly do a full rotation.

6) At this point, holding the instrument so that the valve can't fall out, it should be possible to blow through the instrument. If not withdraw the valve a little until it is possible to rotate it and turn 180 degrees - lower as you do so, it should click in and not turn anymore at about 180 degrees. You should be able to blow instrument. 

7) Carefully screw on the valve cap.

8) Go back to 1) for the next valve.