Kind of a tough question and just going to answer it with my limited knowledge based on what you want to use it for.
First, to get the nicotine, you can simply soak
tobacco in water. I don't know what kind of flavor you will get as flavor extraction will be covered in a minute.
You will need a kit to measure the nicotine levels but after extraction in water, let the water evaporate off. I would suggest distilled water for this as you will need the nic extract to hold some water. I don't think you would want to use nic base in paste form. Just my opinion. I'm sure there will be some flavor but what flavor profiles you will end up with is unknown to me. Let's just assume for now that they will be water soluble flavors.
So, extract, evaporate, test with nic level test kit.
I have had success with flavor extraction using the PG and microwave method in this section of the forum. Soak the ground tobacco to be used in PG and nuke for 5 seconds for a shotglass sized quantity. Adjust time for amount. Then macerate this heated mix and nuke for another 5 seconds. Macerate again and let it sit for 15 minutes. Strain then let this mix sit for about 3 hours. Mix to your preferred levels of VG/PG and test the flavor by adding your PG extract. Often, you will need this mix to steep for at least a week (or longer) to get the taste where you want. THEN, add your tested nic extract to proper levels for you.
Now, this is a very simplified layman's method. I'm sure there are much more experienced people here who could chime in to correct me where I'm wrong as I'm sure I am and probably leaving out some very important steps. SO, I am only suggesting this as a first step to start your READING research into how to do it correctly. I also post this in this manner to suggest that you do not try to extract nic and flavor in the same process together as you did state that you wish to avoid other chemicals not are usually not present in prepared extractions/flavors from vendors that are not listed as containing more than just nic and VG/PG.
I would appreciate all input and enlightenment on this post.