I have looked at the premade logs Keith but personally I like customising things. I think I may use both paper and technology. Sort of do a rough draft in a book on the bank then perhaps type it up at home later. Mark's last comment about using Microsoft excel has really give me something to think about.
Just to give you an idea Joshua, my headings are day, month, year (do them in separate columns!), wind dir, wind speed, weather(which I convert into a number, 1 for sunny, 2 for overcast etc), air temp, water temp, pressure, water colour, moon phase (1,2,3 and 4!), notes, bait, venue, species caught in separate columns, carp, tench etc., size roughly.
Once you have built up a few records you can produce graphs on excel as well. Say a tench graph for venue X per month/temperatures, weather etc or carp different boilies, roach and air pressure ranges or whatever you want really, no end of permutations you can play with..
You don't have to do the same as me, just to give you some ideas, as you say tailor it to what you want. the thing is get it right because 5 years down the line you don't want to be thinking wish I had included/done that. I remember I thought after a couple of years of doing it I should have included moon phase, would be interesting just to see if it means anything, fortunately in this instance I found a table online and just used my dates to go back and enter them in a column. However, I wish I had been a bit more specific in the baits I used and I couldn't do that with memory.
And I think your right, keep a more personal dairy type and an excel information data one separately, I wish I had done that come to think of it. But the excel option does become interesting once you have built it up, so much you can do with data on excel. Written in a diary it can be just a blur as Artist said but excel can specifically define any data any way you want.
It takes a bit of getting used to but its a lifelong project and gives you plenty to do on a cold wet afternoon in winter and anything that makes life/fishing more interesting is worth having a go at-good luck..
PS-just another tip, when I started I thought I do not want to just enter 5 roach when they could be 5 1lb+ roach or 5 4oz roach. Then I thought I don't want to be weighing all the fish, I would have to shoot myself. So, I used a simple scoring system, in my notes I would mention 2 very good roach, 1 average roach and 2 small roach. but, in my roach column I would score 15 for each of the very good roach, 10 for the average roach and 5 for each small roach giving a total of 60 points. This would be dependent on the average size of fish for the venue. So, you have a general score for how good the roach fishing was for that day at that venue. I do the same for all species. Not to be pedantic about it, if I caught 40 roach I would just get it roughly based on my memory on the day. You may have your own ideas, something better even but just a suggestion if you go down this route because sometime you may want a score of some sort to be able to define your days fishing and make it excel comparable for other days and different venues/conditions ect for each species, memory fades.
And if all this sounds a bit convoluted and hard, its not once you get going, takes me 5 minutes; once you get your system sorted out; no hardship at all.
