Welcome to ECF and modding!
I won't go into soldering unless you ask for more info, since you're already researching it and have someone to help you.
Parallel and series in a circuit are ways of connecting parts. Series is one-after-the-other. Like railroad train cars. End-to-end. Parallel is like parallel lines... side by side.
So batteries in series are like in a standard flashlight. The button ends all face the same direction and the + side touches the - side of the battery above it.
Batteries in parallel would be ... having all the buttons on the same wire, and all the flat (-) on another wire.
I'm not drawing a diagram tonight. But I will describe the connections. Should be fairly strait forward. However, before I do I need to talk to you about 5 volt PTs and your amp question.
The PT will "draw" as many amps as the resistance (pressure valve) of the atty/carto allow at a particular voltage. Now, that # of amps often exceeds a computer's USB output. Also, why tie up a computer USB port with an e-cig? Now the max current is a function of the power source and the cable. There are different grades of USB cable and different power sources.
You need to get a 2 amp or better A/C to USB adapter....AKA "wall wart". There's no battery for the type of PT we're talking about here, so hooking it up to a computer is not advised. If the USB cable you are using is too long, or has too much resistance, it won't perform well. (length of a wire increase resistance).
Now with all that said... If you take a USB cable (thicker = better) and snip the end off and expose some wires you will find there are 4 wires. You need to find the positive and the ground (negative) wires. Often Black and Red. There are two other wires that are data wires. You don't want those.
You'll need an enclosure also. Try something like a 1xAA box. You also have to find an atty connector of the type you want (like 510 or 808). Lastly you'll need a switch (button). Now, it should be a normally-off, momentary on button... what you think of as a normal button....pushed = on/closed, released = off/open. It should be rated at 3 amps or better if you can manage it. The higher the rating, the longer it will hold up. We're going to pass all the current through that poor little switch.. for this version of a PT. If you want to get fancy, research power-fets or MOSFET. However, that's more than I want to go through for a newbie.
You'll need 4 properly sized and positioned holes in the enclosure. 1) Atomizer Connector. 2) Switch/button 3) USB cable. 4) LED
After you drill the holes, test fit all the parts before you solder. More on the LED in a minute. For now we'll concentrate on 1, 2 and 3.
Put the cord through the bottom hole. It's not bad to put some strain relief on it. If you wish, you can use heat-shrink tube. Or you could use tape. If you use heat shrink, put the outside tube(s) on 1st, then put it through the hole. Then put the inside ones on. Don't shrink any of them yet though. Now run the cable through the atty connector hole. The cable is now through two holes; bottom and top, and running out the top. Now you can solder the wires onto the atty connector and slip the connector back into the box when done. If you don't run the cable through 1st and just solder to it you're hosed since you can't get the cable into the holes afterword.
Solder the negative (black) wire onto the shell of the connector. I like to do it to the inside so I can still slip the connector into the hole. The positive wire goes to the center post.
Let it cool down for a min or two, then slip the wire and connector into the hole. Add the switch to the box and screw down the retaining nut or whatever it has to hold it in. The switch goes on the positive wire IMO. Could go on the negative wire. It does NOT connect the two wires. It's one or the other. So let's assume we put the switch on the positive wire connected to the center post. Snip that wire above the switch and solder it to each pole of the button/switch. Pressing the button connects the wire and makes a complete circuit through the atty.
This is also when you would shrink down the heat-shrink tube to provide strain relief for the cable. It would basically lock the wire into position around the hold and not have it tug on the switch connections. You could also put tape around the cable inside and outside the hole to do the same thing.
Now you have a working PT and can test it out using the AC adapter and an atty/carto. Plug it in and you should get sizzle. If not, check the wires and see what works. Unplug it and snip the two you don't need.
If you want to add an LED, you need to have an LED and a resistor (guessing 440K ish..varies by LED). LEDs only work in one direction, so you have to get the polarity right. Current only flows one way in them. Solder the resistor to the LED's longest lead. Now touch the resistor to the atty connector's shell and the other lead (no resistor) to the switch ON THE ATOMIZER SIDE of the switch. Press the button. It should light. If not, switch it around the other way and try again. Once you get it to light, solder it in place making sure you get it positioned in the hole made for it and there are no shorts with those bare wires. You can use some electrical tape, heat-shrink tube or epoxy to insulate it.
Now mix up some epoxy, or use some hot glue, and glue everything in place after giving it one last test. Make sure you seal all the way around the atty connector. Let it harden. Close up enclosure.
Just as a note, unplug the thing from the power supply while soldering. I'm sure you knew that, but it's easy to forget so I mention it here too.
Congrats. DIY 5 volt "direct" PT.
Reminders:
Don't use on a computer's USB port.
Get a wall wart with enough amps to handle whatever you have planned. 2 amps is OK for normal (3 ohm) stuff. If you want to use a DCC at 5 volts, you'll need at least 3.5 amps.
Don't leave it plugged in when unattended (IMO).
If you want to add a MOSFET, they will reduce the voltage since they all have some drop out. That's beyond the scope of this, but LDO is what you are looking for and pay attention to amp ratings and gate types.
