I found lay betting really confusing at first. I was thinking it's like back betting, and couldn't get why betfair wanted 30 quid when the bet at William Hill is £10, the way I figured it out is realising that the lay bet is more because you're kind of paying the winnings out to someone's back bet. It's really scary when you first start.
The higher your back odds, the higher your liabilities will be. A lay against a £10 back bet on odds of 2 will have a much lower liability than a lay against a £10 back bet at odds of say 7.
There's a calculated on oddsmonkey which tells you exactly what you need to lay, at what odds and how much in funds it will cost to place the lay. Profit Accumulater also has the calc available.
I understand why some may not want to pay monthly memberships for sites, I didn't want to and tried doing the free trials and then doing it myself but I found it overwhelming, looking for bets and offers myself and didn't feel confident I was laying correctly, the premium membership has been well worth the money and I regret not signing up sooner, it is possible to do this yourself without premium membership but I do think you need to know what you're doing and a grasp of how the lay works. I'm absolutely shit with maths and it's just a matter of time until I mistake on the lay, (many of us do)knowing I can get help on how fix a cocked up lay from support staff makes that £15 a month well worth it, plus it takes a fraction of the time to find deals and bets.
Most premium sites give you a free trial where they guide you through two offers, step by step, in the first two offers you've earned enough to cover a months membership and then if you feel confident you can cancel the membership. The membership more than pays for itself in my opinion. I use oddsmonkey and have been going since mid April, I'm still working through guides and have plenty to learn so as long as I make the monthly membership in profits I'm keeping it going. Profit Accumulater is another popular site.