I run a pretty successful RS Skilling service atm (though I promised myself I will NEVER charge for my services), and basically all you have to do to start is offer to help. I would recommend getting 4/5 "supporters" or "vouches" that prove you have done a service of some kind (in this case, fishing) efficiently and did not take the account.
I would recommend not charging "typical" prices until you get about 10 vouches. I have done 8 jobs for people, but I've been given level 100+ accounts to work with - and those vouches count a lot. If you're able to keep an account safe and secure while giving someone free levelling, that's basically what people are looking for.
Here's a simple process I would recommend doing:
1) Create a MSN account. Make it very secure.
2) Go to
http://www.sythe.org/forumdisplay.php?f=445
3) Create a new topic. Make your title something along the lines of:
~100% Fast, Free Fishing [and other skills] from Ezp~
4) In your post, mention how you are a new member but are looking to build up some trust and respect, and that you'd like to help people out. Try to keep a GOOD lay-out, talk about yourself a bit, etc. Make yourself appealing to use.
5) Someone will be desperate enough to need to use you, I guarantee it. When they do, make sure you don't let them down. You always want the first post to be a positive one, rather than a negative. Ask them to post their MSN, and contact them. This is a good way of protecting yourself from imposters.
6) After you get the vouch/respect from this one member, post it in your thread. More people will come - I've found that the growth is pretty much exponential. Just keep your thread organized, and continue doing what you like.
7) If you want, you can post on your thread that you're doing the first 10 accounts free, then after that you will begin to charge. At first, charge about half of what other people on Sythe are charging [AFTER you get 10 vouches for free services], and slowly work your way up!
*note* if you want, you can always just continue doing free services [like me!

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I hope I was able to help ^_^