Building rails is the best place to start when learning to work with logs, the work is repetative and you will soon master the art of making spindles and rails. The most common size for deck rail is 6" posts, 4" rail & 3 1/2" spindles, rails have 2 1/4" tenons and spindles 2". This is the most common but some people like 12" rails while others like 2" spindles, so it's good to be able to do everthing in between.
    When building deck rail the most important thing to keep in mind is the deck that you will be attaching to, if the deck won't support the rail it won't matter how well the rail is made it will not be stable. The deck should have a minimum of two joists all the way around and be built secure. Take care when collecting your measurements as there is very little room for adjustment, if your measurements are precise then the rail has a better chance of being tight. The tenons on the rail should enter the post 2 1/2" and when measuring the top rail it is good to take into account the taper of the post.
    The easiest method of securing the posts to the deck is to notch the post so that half of the log sets against the joist and the half that was notched out sits on top of the deck. By doing this the center of the rail will be right at the edge of the deck. The one thing to look out for here is that the deck floor matterial does'nt overhang the joist, if it does then it must be trimmed back or notched out. The post is secured to the deck by two 1/2" bolts, lag bolts can be used but are not recommended. It is better to have a nut and bolt in case the rail loosens up over time, it can be easily tightened whereas with a lag bolt it is too easy to strip out and then you have a problem.
    Spacing is the most difficult part to master, when using a 3 1/2" spindle and you want a 4" spacing you want to drill the holes at 7 1/2" apart. Sounds easy, but it seldom works out, so the first thing you do is take the measurement between the posts and subtract the first spacing on either side. This would be 4" + 1 3/4" ( half a spindle ) X 2 Then determine the number of spindles required by dividing the distance by 7.5 Next divide the number of spindles into the distance to give the exact spacing. For example the spacing on a 70" rail would be 70 - ( 4 + 1 3/4 ) X 2 = 58.5, then 58.5 / 7.5 = 7.8, round off to 8 spindles and divide into 58.5, 58.5 / 8 = 7.32" This is roughly 7 3/8" and the first spacing can be narrowed up to 5 1/2 this will now work out to equal spacings of 7 1/4". If this looks complicated it's because it is complicated but it is worth the bit of effort to figure it out. Spacing is the first thing that most people will notice if you do it wrong, the good thing is that it is very hard to tell the difference between and a 7 1/4" spacing a 7 5/8" spacing while looking right at the two side by side.
    Stair rails present even greater difficulties and I will add a page devoted to that at a latter date.