Well.....make sure to unplug from electricity first, and heed the cautions about high voltage being present inside the chassis. Even when unplugged from electricity, the capacitors can still hold a lethal charge. Best not to touch anything inside the chassis, except for test points and adjust pot.
The way that I do it:
Remove the top and rear metal screens (grills). Remove the output tubes, noting their position, a sharpie is good to mark the tubes (4 and 5). Remove the four screws on the bottom of the amp that hold the chassis in (not the feet, the other four screws), making sure that the chassis is secure and won't fall, ie leave the amp in upright position and undo the screws from underneath. Hang one end of the chassis over the edge of workbench and remove those two screws, then do the same for the other end. Remove the chassis from cab (grip the transformers if necessary), one way to position it as shown in pic (above). Replace the output tubes, connect amp output into a speaker cab, connect DC volt meter to amp test points (read mV), plug amp in to electricity. With all knobs set to zero (not sure if that is necessary but I do it anyway), switch amp on, and after a minute switch standby on and note mV reading at test points. [Edit: if installing new tubes, make sure that the initial mV reading is not excessive, be prepared to turn the bias pot down if necessary]. Wait a few to let tubes warm up. Adjust bias pot to desired level (I set mine at 78 mV), then let the tubes warm up for a while, maybe even plug in a guitar and play for a few minutes, recheck bias before replacing the chassis back into cab, as a little drift might occur.
Once bias is steady at the desired level, switch to standby for a minute, then switch the amp off. Unplug all connections from amp and reassemble in reverse order of dis-assembly. Last step: enjoy!