I've been leading worship for quite a few years now and have never led with an instrument so it really doesn't have to be a hindrance.
Here are a few thoughts I hope you’ll find helpful:
You don't have to be a star musician on an instrument to bring the best out of your band. What you can do is come to the rehearsal fully prepared. This means you worship with and think through the songs before you get to the rehearsal, so you have a clear idea in your mind as to how you think the arrangement will go, and the dynamics that you’re hearing and so on. Therefore, even if you can't execute it on an instrument yourself, you can give the band some clear guidelines of what you have in mind.
If you don’t know any musical terms, just use adjectives such as "I'd like this to be majestic" or give an instruction such as "let's have a strong introduction then drop down a little in verse one, building to the chorus" or invite the band to add some interest in verse 2. Think it through: do you think you’ll do a single bridge, or perhaps repeat it? If you’re going to repeat it, invite the band to perhaps hold back the first time through so they have things to add on the second time through. After the bridge, will the chorus drop down or will it stay big? The more you can think these things through at home, the more confident you will feel in the rehearsal. Just try to avoid using exactly the same arrangement for every song!
It's important you have a sight line to all the members of the band so they can see your signals and that they can hear you clearly, so always check that they can hear you in your soundcheck. The signals I use are a V shape with my fingers, pointing downward, for verse; a C shape for chorus, my hand held downward in a fist to signal the end of the song; a little mini C shape to signal a turnaround where you might repeat a line. I have one odd signal for the bridge or Middle 8 which is that I discreetly point to my bellybutton (it's my 'middle') but feel free to make up your own one! If I want to go to just drums, I point to the drummer. Or if you want a particular instrument to play out (electric guitar, piano) you could point to them, too.
In rehearsal, I will usually run an arrangement for a song with the band which I would call the 'default arrangement'. This would probably be something like Verse 1, Chorus, Verse 2, Chorus, Bridge, Chorus, Chorus. But the band understands that I may wish to deviate from that in response to the congregation and to what God is doing among us. That is easily done with hand signals and also just by speaking out, "let's sing that verse again" and so on.
Make sure the band is looking to you for a signal at the key moments when decisions need to be made, for example, are we repeating this chorus? You need to be clear and decisive with your signals (give them, perhaps, a line before the section is going to end) and they need to look up and see them. This is a good thing to practise in rehearsal so you can all get used to it.
If they're not looking at you but you want to change from the default arrangement, then you just need to call it out clearly so both the congregation and the band know where you are going. I will often give spoken instructions as well as signal the band as I think it can be helpful for the congregation (and the person on words projection!).
The other thing you can practise in rehearsal is any 'open sections' where you might want to leave room for the congregation to sing out freely to God. If I want to signal this I wiggle my fingers - again, feel free to make up your own signal! The thing I would do in my preparation is try and identify a key place in the worship time where I think the congregation is most likely to respond in this way. Then, in rehearsal, I would tell the band that I might open the song up at this point and decide with the band a chord sequence so that everyone is playing the same thing. You might choose just to play between two chords, or you could have a longer sequence if you wish.
If you’re short on time in your rehearsal you can ‘top and tail’ a song which means you make sure everyone knows how it starts and how it ends. You will also want to check everyone knows how to get into the bridge, and what happens in between the chorus and another verse, and how to turn the chorus around if you repeat it. If you’re very short on time you can just run these bits!
Make sure everyone is clear how each song is starting – whether it’s the whole band in together, in which case your drummer will probably count it in; or it could be started by a solo instrument such as piano or acoustic guitar; or you could run two songs together that are in the same key.
You are the worship leader but that doesn't mean that you have to have all the musical ideas. I always encourage the band I'm playing with that although the worship leader is making the decisions, as lead worshippers you're all leading the people in worship together, it's a team effort and their musical gifts are making a huge contribution to that. I like to encourage the band’s creativity – God has gifted them on their instruments so we want to use their gifts. In the example of repeating the bridge, while I might come with the idea to repeat it, I might just say to the band, ‘let’s make the second bridge different to the first – how can we make it bigger?’ and see what they come up with in specifically how to do that.
Remember though, you are still responsible for leading the worship time, so if something is suggested that you really don’t feel will fit, then it’s fine to politely say, ‘That’s a great suggestion, thank you, but it’s not actually where I heard this going. What I have in mind for this section is….” The thing is, though, that if you don’t have anything in mind then you will probably end up going with what was suggested, even if it’s not quite what you what. The point is, some things can be figured out in rehearsal, but if you turn up with no clue you’re going to waste a lot of time trying to figure it out on the spot and that’s not what the rehearsal is for. You really need to be more prepared than that.
The worship time itself
Much of the same follows on from the rehearsal. You need to make decisions in time to communicate them, and you need to signal those decisions clearly.
If something changes from your ‘default arrangement’ or the way you rehearsed it, you can lead your band anywhere as long as you are clear on your intent. If the congregation really catch hold of a chorus and you can feel like you’re ‘riding a wave’, just go with that and you can easily indicate that to the band by either speaking into the mic or having a signal that indicates ‘keep going with this’. You don’t want to miss an opportunity for the church to have a real breakthrough in praising God just because you didn’t rehearse it that way! We always want to ‘keep in step with the Spirit’ (Galatians 5:25).
Perhaps you’ve had a rushed rehearsal and didn’t get through everything, and then you’re in the worship time and you can tell the band is all having the same thought – ‘how does this song end?’ It’s going to be up to you to bring it home. There’s no need for an unconfident ending if you just make a decision and signal it clearly. Your choices will be 1) playing a musical outro (in which case probably point to the electric guitarist who will be playing the riff; 2) repeat the last line maybe three times (in which case use your signal to repeat a line; or just go for slowing down the ending (in which case I’d be making eye contact with the drummer and indicating that with my hand). Just because you didn’t rehearse it, doesn’t mean you can’t pull it off.
Don’t be afraid to make eye contact with your band, how else will you know whether they are clear on what to do? If they’re not clear, they’ll be guessing, and they might all guess a different way! If you don’t signal, your drummer will become the worship leader in terms of ending songs and so on. Be brave, be clear, and enjoy worshipping God!