A good idea is if you have the possibility to take some courses in patterndrafting and sewing construction, then you will be able to do everything by urself.
Its good to have some knowledge if you let other people produce for you too.
All the tags and patches and buttons etc.. u can order online, from north america or china, just make ur design in illustrator and pay them... I did some tags in canada it wasnt that expensive.
To make a basic jeans sample wont take that much time when you know all this,
I dont know where you live, like if ur in sweden I can help you.....
The hardest and more expensive part is if you wanna add embroidery and other stuff to them. I suggest you find some embroider in your area and talk to...
If you want print designs on them like evisu have in the back its more easy to do by urself. Just buy some good textile paint, then sand paper the denim where u wanna paint untilit get a light color, now ur ready to paint ur design, if ur good u can freehand,
if not u best bet is to make an template on the computer and print out, then cut out ur design and use the template when ur painting. If you used a good paint and did this properly, the print will really never fade away, doesnt matter how many times u wash them.... the good textile paint color the threads and get into the whole fabric piece...
U will find someok denim in ur closest fabric store...like 5-10 bucks a meter you get a good one, u may need , 2m is more than enough for a pair, make the calculation u find out there a shit load money in the clothing industry
If your in canada i can let u know some people that can help you, then I know a really good factory in china that wont cheat on you, but they may be expensive, theres plenty of counterfeitings factories tho that gladly will help u produce small orders, but they maybe wont do the best top of the line job....