You don't want to do a fire type deal of a forest fire, okay?
That's just going to fucking bring authorities AUTOMATICALLY to the site. And they have to investigate foul play when they just find a fucking body/ashes at the site of a forest fire.
You really want to know your best bet?
Pick your weapon. Knives are good if you know where to hit. I'd personally go for the carotid arteries, the radial arteries, or if you're a retard just jam it into their chest a few times until you puncture their lungs and they suffocate. Guns, steal one. Don't buy one, okay? Even if it's one off the street, they'll just trace it to the last known person who had it and they CAN build evidence back that way. Personally, I think a crossbow/bow and arrow type of deal would be nifty. You can take the arrow out of the body easily, it's silent and efficient. You just have to make it close range, not that difficult eh?
Now. You want to make sure NOONE sees you anywhere near them. Have an alibi. Better yet, have 3 or 4. Receipts work exceptionally well. Ask me if you need to know how to do this, but it is fairly simple. Once you've got that and you've figured out a way to approach the target without anyone noticing you there, you're gonna need to subdue them. You don't want to just kill them then and there. That's far, far too sketch. Drug them up with a hypodermic needle filled with your choice of potent sedatives. Do all of this while taking precautions of your DNA, that means wear hairnet, long sleeved clothes, boots, etc that you will dispose of later. An added point, just in case someone does catch a glimpse of you, alter your appearance. If you don't normally have tattoos or piercings, buy some fake tattoos and piercings and put them in noticeable places. if you wear glasses, get contacts for the job. You catch my drift, I hope.
Now, you've successfully obtained the target hopefully without being seen. What now? My advice *if you use your own car

* Would be to line the trunk of your car with black hefty bags prior to all of this. Tie them up just in case, strip them of any cell phones, and throw them in. Drive like you'd drive if you were already on the FBI most wanted list. Very, very carefully. You want to drive to somewhere already planned for, remote and where noone will look. My suggestion? Mountains. Somewhere so forested and desolate that the only company this place sees is the very occasional hiker or hunter, and that's being pessimistic in assumption. Drive close enough to that location that you can remove them from the car and take them further in. My suggestion also is to have a trunk you can work with, you know a luggage trunk. It'll come in handy later. Now you're out in the middle of nowhere with a luggage trunk, a drugged up target, and a weapon. Do the math. Kill them. Put the crossbow/knife/gun to work and kill them. If you use a gun, I'd take the time to dig the bullet out but that's just me. Next, you're gonna be getting messy.
You want to dismember the body. I'd start with the most important areas, those used for ID. Teeth, fingers, etc...Pull the teeth out with pliers and place them in a container, same with fingers and such. Cut the arms and legs off, cut the body up into pieces and place all of the pieces in the trunk. ALSO make sure the trunk has been lined with hefty bags. You don't want leakage

Dump some chemicals like bleach or ammonia in the area to help remove the blood traces. Next you're gonna want to go to ANOTHER remote location, one where you preferably have access to a metal tub or something like that. One by one, put the body pieces from the trunk into the tub and dump lye and water all over them. Use other acids *Be creative or ask for the best to use* after removing the water from the tub, since water neutralizes acids. Keep doing this until you're down to bones. Do it with the fingers too. Put all of the bones back into the trunk, or find some way to grind them up into dust so that you can dispose of them easier. If you can't grind them, I'd recommend visiting some remote lakes and just dumping a couple bones here and there until they're gone. Sounds a lot harder than it is and when you're wagering your life in prison, it's worth the time and effort. As for the murder weapon, you wanna dispose of it in this way too. Be creative. If you used a knife, melt it down and dispose of it. If you used a gun, file down the barrel and disassemble and just throw pieces here and there at different places. Crossbow, same concept. And the teeth, once again same concept.
Now you're coming to the final phases of your plan. Let's see. You've obtained the target. You've killed them. You've removed flesh from them and blood, making it neater. You've disposed of the bones and any remaining traces of them. What next? Well, now you get rid of anything you used to do all this that you haven't already disposed of. Any clothes you wore, hairnets, shoes, etc, burn them. Obviously burn the hefty bags and shit too. Luggage trunk, throw plenty of bleach or other chemical agents in there to clean it, THEN burn it. Clean out your car with a passion, you've got to remove any traces of those things you used from it. Return home, and never ever ever speak a word of everything you've done to ANYBODY. I don't care if it's your fucking brother, you don't mention it. Keep low for a while, don't do anything stupid. If you've done things right, there won't be any cops knocking at your door. And even if there ARE at some point, DON'T SAY ANYTHING. They'll try to convince you that they've got evidence and all that shit. They don't and never will. You got rid of everything, remember? They can't make a case without a body and/or a murder weapon. And that body and that weapon happen to be in multiple places at once that they have no clue about. Throw your alibis at them only AFTER they ask you about those specific times, and be very vivid in your descriptions of the alibis. Research shows that people who are telling the truth tend to throw more "minor' details into a story than people who are lying. If you jump the gun and show them receipts to say that you were somewhere at the time of the murder though, BEFORE they ask you about the time of the murder, you've pretty much just given yourself up. How would you know what time, if you weren't involved? Just keep quiet, they don't have a case. You can go home safe and sound.
tl:dr if you're honestly too lazy to read and apply all of that, here's an alternative. Steal a low level dealer's gun. These are usually your typical dumbass weed dealers who will keep their handgun in their car. Now, find your target and figure out their routes. Disguise yourself with fake tattoos and all that shit, hide your face, pick a nice alleyway or something and wait. Find a way to lure them into a deserted area, that's the idea, and shoot them or kill them somehow. Steal everything of any value on them, you want to make it look like a robbery, not a planned murder. GTFO the area, dispose of clothes and weapon. Much riskier plan, yes, but much faster.
Here's yet ANOTHER small plan, kinda came from a brainfart. Let's say you know enough about their lives to know who they're close to. Buy a couple burner prepaid phones WITH CASH. Find some way to get one of the phones into their hand. While watching them from a distance, call them from the phone you kept and tell them you've taken their loved one. You're going to set up a meet. Tell them to meet you at a location, and to not deviate from the plan at all. Tell them to take out their own phone and also the one you gave them and throw them, so that you can watch them and make sure they don't contact anyone. Lead them somewhere remote. Have them stand there, waiting for your contact. Shoot from a distance, you can figure out how to do that on your own. Destroy evidence and remove phones.