There is an excellent article on using Galdr (invoking through sound/incantation the attributes of the rune) for runic spells and magic at http://www.sunnyway.com/runes/galdr.html You might find that useful
Along with the suggestions given, you could go about it a couple more ways. 1) you can simply say the name of the rune as a method to invoke it. Though this is a more modern method and is the weaker, unconventional way - per say.
2) if singing the rune like "fehu fehu fehu f f f f f f f f f fu fa fi fe fo of ef if af uf f f f f f f f f f" doesn't quite fit your fancy, you could go about it by singing the name like you would words to a song. Fffeeeeeehhhuuuuuu. Giving extended emphasis on the vowels of the name. As I said, much like dinging words to a sing.
I don't recommend speaking the name of the runes, for it is weaker and not really as it should preferably be done. The vibrations from singing the name is what you really want. This gives a more - fuller - potency when invoking.
To to add to this, there are some other methods to incorporate: holding the rune in your hand; drawing it whether that be on your body, on the ground, on paper, or painting it on a surface; and mentally visualizing the rune; all of which can be whilst you invoke the rune.
If you really want to get into the nitty-gritty of Galdr, the most traditional and "powerful" way that Galdr was used in regards to runes was impromptu inocation-"spellwork", much like how a bard or poet strings together a song, but with the invocation of the runes included. The idea of runic-magick consisting of divination mainly, or the literally singing and phoentical-sounding out of the rune, is actually quite modern in comparison and I wouldn't say necessarily "stronger" or more "potent". However, I would say often times it is more simpler. If you're going that route about it, there are other things you can include as Vanitys suggested. You may want to look into Stadhagaldr, or runic yoga, and assume the fehu position while chanting/singing/rhyming, etc.
May I ask why you're invoking fehu? Since the others summed up how to, I'd like to explore the purpose. Many times it is best to invoke more than one rune to guide the situation properly, and fehu may not even be the best one for your goal. Take note, the meanings behind runes in magick use are different yet similar to those used in divination.
I know this is an Old one but I have added suggestions.
In my studies Fehu has many meanings. When I picture Fehu it reminds me of the day that Odin walked to Yggdrasil and perhaps the first day of the nine which he hung on Yggdrasil. Before summoning Fehu perhaps mediate get to know Fehu as Fehu is and how. Once you find common ground you may find have. Perhaps rad poems of fehu from old to modern times.
