I think it’s like with everything else. just do it, look in your room for an object or somewhere else and just recreate it and if it doesn’t look good, do it again. But you also have to be honest with yourself and be aware of your skill level. If you’ve been learning for a few weeks, I wouldn’t recommend a complex mesh because it will be too frustrating.
I have learned that the challenges then come naturally. The man “Wes Burke” started a weekly challenge here on CGCookie a few weeks ago. Simply go to Community tab on the site and then set the filter to CGC Modeling Chalenges.
And don’t put yourself under pressure, you don’t have to be the best straight away. Just do it, have fun and the progress will come naturally. What I learned recently. Slow means accuracy and accuracy means speed. I think that applies to a lot of things.
Happy blending
