Goodness, I have no idea why no one has posted an answer yet.
For starters, try translating texts. Take your time with coining new words and grammar. Think about how they will sound later on. If you just rush through, you risk ruining the conlang's original euphony for yourself
If you're stuck with grammar, don't be shy to take a piece from any grammar from any language in the world. Just don't make it too kitchen-sink.
If you're stuck with word coinage, consider 1) creating derivational suffixes to attach to existing roots and 2) conjoining roots into compounds (if you run out of imagination, check out the etymology of natural compound words. For instance, if you're making a word for "etymology", look at its etymology. It derives from étumon (true sense) and logía (word). That should jog some creative juices).
That's just a rudimentary guide based on my own experience. I would love to recommend Mark Rosenfelder, but unfortunately, his guide didn't help much when I was stuck (I use the book, by the way). I read that thing from front to back, but, aside from a really huge section on pragmatics (which I hadn't thought of before but couldn't really act upon when I did), there was nothing to shift me out of my rut. Unless, of course, I'm just a terrible reader.
Fluent: English, русский язык
Proficient: français, 日本語
Beginning: suomi, davvisámegiella, 中文 (普通話), norsk, cymraeg