If you haven't yet had any paid commissions, what should you fill your portfolio with?
Magazine mock-ups. Most new illustrators will get their first commissions from magazines, and a mock-up is the easiest way of showing a client what your work would look like in the context of a magazine like theirs.

Some balk at the prospect of replacing a photo or someone else's illustration with a piece of their own work but it's common and perfectly legal, at least in the UK. Plus, if you're not used to seeing your work alongside type or other images, it can prove educational to the illustrator as well.

Word to the wise, however: when showing mock-ups don't feel tempted to show the piece you are replacing for contrast purposes. The client might decide they like the other illustrator’s work better.

How important is presentation, do you need, for example, a premium-quality portfolio or will an entry-level one do?
Presentation is highly important – in as much as you want something simple, clean, lightweight to carry and easy for a client with limited space and limited time to negotiate.

If, by entry-level, you mean a foundation-level cardboard job secured by pieces of string and held under the arm, the answer is definitely no. Likewise cheap, tacky, little folders with fixed plastic leaves. However, a standard zip-up portfolio with ring-binding is absolutely fine, as is a soft-cover folder with ring-binding, held inside a zip-up carry case.

Some illustrators favour boxes with loose leaves inside though, personally, I would worry about pieces of work straying during a drop-off presentation. It is possible to pay hundreds of pounds for a hand made, leather tooled, whistles-and-bells portfolio but I don't think it's necessary to fork out that kind of money. A brightly coloured, idiosyncratic or gimmicky portfolio might stick in a client's mind, but, ultimately, it's what's inside that matters.

If you favour a virtual presentation, don't rely on being able to use a client's computer or assume they’ll have Wi-Fi. Always bring your own laptop, or one you’re familiar with, plus power back-up, a memory stick or work on disk, and even a few printed pieces just in case.

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