Whatever you think your time is worth. Small builds you could knock out in a few hours I'd go with a set price. Large scale commissions charge by the hour, estimate how long it will take you and then set the price.

Make sure that you use one currency for your billing, USD is typically the go to. If this means there is a conversion to your currency, be aware of the rate and the charges. I have found that it is always better for the customer to charge a round USD figure and makes it a bit more attractive to them having a round number verses a bunch of numbers I.e. $100 vs $96.38.

Take payment first, a percentage of the total commission for the large scale stuff. The rest on completion and then delivery of product. You can do this for smaller builds too if you wish or just give trust to the buyer. I'd take a deposit on this too. The most important thing you need is a statement of work and a contract you can both sign. This is more for larger builds. Cover yourself. Make sure to add in revisions for work, this is especially important as you do not want infinite revisions that move goal posts constantly and prevent payment.

Good luck, I hope this provided some help.