So I have a guitar with 2 EMG-HZ 7s in it and I really dislike them so I decide to buy 1 EMG active pickup to replace the bridge HZ since I use that one more.
So the first thing I do is get the strings and bridge off. I unlock the string locks (you might not have them) and loosen the strings.
I then say "hey the bridge is whack" so flip the guitar around.
After taking off both plates I take my pliers, grip the end of the springs and pull them out one at a time.
I flip the guitar back around and take the strings off:
Now I take a look at the guitar and decided that the frets were kinda grimy so I grab my trusty 0000 steel wool and painters tape and do like this... note that some guys do up the whole neck at once with the tape, that's a waste of time, just use 2 pieces on either side of the fret you're working on and replace them as they are worn through.
Enough of this foolery! I flip the guitar back around and find which lead goes to the bridge pickup, in this case there are 2 holes so it was obvious which was which, it's not always like this so sometimes you'll have to take the pickup out first and pull on the lead to find it, then put it back in place and desolder it.
I look in the electronics cavity and find the tone pot. I only have 2 pot holes in my guitar so I have to use 2 volumes instead of volume and tone for the active-passive combination.
I then turn the guitar back around after desoldering the bridge wires from the switch (this step is easy, look at the switch and desolder both things connected to the bridge wire)
This is the HZ taken out, note the foam pad things, we need those.
This is the HZ and 707 next to each other, I'm showing my love and compassion for the HZ. Notice I took the pads off the HZ and the jack location on the 707, we have to cover that up with the pad.
Now this is the 707 with the jack attached (their diagram is kind of confusing, this is how it goes on) and with the pads put in place. If you weren't lazy you would cut a little hole for where the jack goes but it doesn't really matter too much unless your cavity is really really shallow.
I then flipped the guitar back over (without puttin the pickup in) and unscrewed the old jack.
From now on ignore where the wires on the pot and switch go because I screwed them up, but the technique is good.
In this picture I moved the original volume pot (for the neck pickup now) to where the tone pot was and put in the EMG volume pot which I had to desolder the tone pot from. (actually I just clipped the leads because it was easier).
Notice that I desoldered basically everything except the ground from the bridge (which I leave connected even though EMG says not to) and kept the part which was connected from the middle terminals on the switch to the volume pot, we connect that back to the switch but to one of the side terminals.
I then installed the jack from the EMG package but haven't connected it yet. I soldered the volume pot for the HZ (original one) to the switch position for the neck pickup.
I next 'tinned' the wire for the EMG pickup. This just helps a bit when soldering, it doesn't really help us much here but I'm used to doing circuit boards and this is how you do them.
I soldered the tip of the wire to the lug on the pot (this should be on the middle lug or where it is depending on how you wire it).
I then scraped the back of the pot with my screwdriver, applied some solder to it and soldered the braid of the wire onto the pot. This is the ground.
I skipped a bunch of pics for the next few steps because as I said I put the thing in the wrong position.
Connect the lug which shouldn't have been used in the last pic to the bridge position lug on the switch.
You saw the white wire connected to the middle lug on the 707 volume pot before and you must be wondering, what the hell is that, well that was the connector to the jack, you connect that to the middle lugs on the switch, if you were to have 1 pickup it would go where it was in the pictures.
I labelled this picture with colours as to where the wires go and what they are. The green is the positive leads to the jack, the black are the ground leads to the jack, the blue leads are the signal from the pickups and the orange around the perimeter of the picture is the battery connector from the jack to the 707.
After that's done, put the guitar back together and then check out the springs for the trem. I took some of the foam from the packaging, cut it up and stuck it under the strings to dampen their vibration when I'm playing. I then took a big block of the foam and put that on top of those in-between them and the plastic cover.