I would recommend the Premium Leaf. As a rule of thumb, the Kratom in headshops is often stale, commercially-produced downright weak and useless.
Premium leaf has the full-alkaloidal content, whereas you are correct, extracts tend to be oxindole extracts of a specific alkaloid, namely 7-hydroxymitragynine, mixed in part with regular leaf.
You need the full range of alkaloids to get the best effect, and this is done by ingesting raw kratom leaves via the toss and wash. IF its superfine (which it should be hopefully) then, starting with an empty stomach (this is the most important, otherwise it simply wont work. Make sure you are totally empty, but have a banana (or ginger-based drink) or something to battle any potential nausea. You can safely eat about 1 to 1 1/2, more towards the 1 1/2 hrs after ingestion.
1) Measure your dose which should be anywhere from 5-15g (if its premium leaf). Now, what type you have will make a difference. Eg, Maeng Da Thai is strong, so a beginner should have about 6g to see how you find it. A Thai strain is more stimulating than Indo's and Bali's. A Premium Red Vein Thai should be about 6-8g as a starting dose, no more. Same goes for indo or bali. There are loads of types, from super green indo to premium bali to enhanced green malaysian (malaysians tend to be more stimulating). My dose varied between 10g to 20g, but for the past year was 10g...12.5g was my dose for a long time too.
2) Dump a teaspoon (I usually find its about 2-3g) of the powder on your tongue and take a swig of your drink. I use OJ as its thick and carries the powder well.
3)Dont swish or anything, just have it in a nice pile on your tongue and washed down quickly and smoothly with the liquid. Use a thicker liquid like OJ or a fruit juice but do not use a carbonated drink, EVER!
If it is crushed leaf, then you could grind it further and t'n'w or if there are no stems, then try t'n'w it.
You can also make a tea, steeping it in boiling hot water for 30-40mins straining, repeating, consuming (or further boiling down to a more manageable amount)
You can stir your dose into a yoghurt, but this weakens it, as do gelcaps.
You can bomb it in skins (place the bottom of a skin along the sticky part at the top of another, to create a 1-skin-length double skin which can hold alot more...i think i managed to fit 2.5g in there.)
You can also make a juice paste. THIS IS EASY if yu cant handle the t'n'w.
Place your dose in a glass and pour enough OJ (or whatever) in to just cover it, maybe couple tablespoons. Not too much. Stir in just enough so it makes a paste, akin to the texture of wallpaper paste, not too dry so its grainy, but not too wet that its runny. You will get the hang of it. Then you just fill up the glass with more OJ and give it a good stir for a minute. Chug.
Mixing your strains is essential, but i found that no more than 2 strains was best. I used to love mixing a relaxing Indo, such as Kali indo at 5g to 5g of RVT. In the morning, for work, it was great.
Any more questions, just ask (i know some great vendors but dont know whether i can give you a link?)